Capture std::clog writes and combine them with std::cerr writes (#989)

This also introduces Catch::clog() method to allow embedded targets
to override std::clog usage with their own stream (presumably null-sink),
similarly to how Catch::cout() and Catch::cerr() are used.

Fixes #989

projects/SelfTest/Baselines/console.std.approved.txt

@@ -622,6 +622,9 @@ with messages:
 
 A string sent directly to stdout
 A string sent directly to stderr
+Write to std::cerr
+Write to std::clog
+Interleaved writes to error streams
 Message from section one
 Message from section two
 -------------------------------------------------------------------------------
@@ -953,6 +956,6 @@ with expansion:
   "first" == "second"
 
 ===============================================================================
-test cases: 168 | 119 passed | 45 failed |  4 failed as expected
+test cases: 169 | 120 passed | 45 failed |  4 failed as expected
 assertions: 968 | 859 passed | 88 failed | 21 failed as expected
 

projects/SelfTest/Baselines/console.sw.approved.txt

@@ -6786,6 +6786,39 @@ PASSED:
 with expansion:
   Approx( 1.23 ) != 1.24
 
+Write to std::cerr
+-------------------------------------------------------------------------------
+Standard error is reported and redirected
+  std::cerr
+-------------------------------------------------------------------------------
+MessageTests.cpp:<line number>
+...............................................................................
+
+
+No assertions in section 'std::cerr'
+
+Write to std::clog
+-------------------------------------------------------------------------------
+Standard error is reported and redirected
+  std::clog
+-------------------------------------------------------------------------------
+MessageTests.cpp:<line number>
+...............................................................................
+
+
+No assertions in section 'std::clog'
+
+Interleaved writes to error streams
+-------------------------------------------------------------------------------
+Standard error is reported and redirected
+  Interleaved writes to cerr and clog
+-------------------------------------------------------------------------------
+MessageTests.cpp:<line number>
+...............................................................................
+
+
+No assertions in section 'Interleaved writes to cerr and clog'
+
 Message from section one
 -------------------------------------------------------------------------------
 Standard output from all sections is reported
@@ -9483,6 +9516,6 @@ MiscTests.cpp:<line number>:
 PASSED:
 
 ===============================================================================
-test cases: 168 | 118 passed | 46 failed |  4 failed as expected
-assertions: 970 | 859 passed | 90 failed | 21 failed as expected
+test cases: 169 | 118 passed | 47 failed |  4 failed as expected
+assertions: 973 | 859 passed | 93 failed | 21 failed as expected
 

projects/SelfTest/Baselines/junit.sw.approved.txt

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <testsuitesspanner>
-  <testsuite name="<exe-name>" errors="13" failures="78" tests="971" hostname="tbd" time="{duration}" timestamp="{iso8601-timestamp}">
+  <testsuite name="<exe-name>" errors="13" failures="81" tests="974" hostname="tbd" time="{duration}" timestamp="{iso8601-timestamp}">
     <testcase classname="global" name="# A test name that starts with a #" time="{duration}"/>
     <testcase classname="#748 - captures with unexpected exceptions" name="outside assertions" time="{duration}">
       <error type="TEST_CASE">
@@ -475,6 +475,13 @@ A string sent directly to stderr
       </system-err>
     </testcase>
     <testcase classname="global" name="Some simple comparisons between doubles" time="{duration}"/>
+    <testcase classname="Standard error is reported and redirected" name="Interleaved writes to cerr and clog" time="{duration}">
+      <system-err>
+Write to std::cerr
+Write to std::clog
+Interleaved writes to error streams
+      </system-err>
+    </testcase>
     <testcase classname="Standard output from all sections is reported" name="two" time="{duration}">
       <system-out>
 Message from section one
@@ -750,6 +757,9 @@ hello
     </system-out>
     <system-err>
 A string sent directly to stderr
+Write to std::cerr
+Write to std::clog
+Interleaved writes to error streams
     </system-err>
   </testsuite>
 </testsuites>

projects/SelfTest/Baselines/xml.sw.approved.txt

@@ -7309,6 +7309,24 @@ A string sent directly to stderr
       </Expression>
       <OverallResult success="true"/>
     </TestCase>
+    <TestCase name="Standard error is reported and redirected" tags="[.][hide][messages]" filename="projects/<exe-name>/MessageTests.cpp" >
+      <Section name="std::cerr" filename="projects/<exe-name>/MessageTests.cpp" >
+        <OverallResults successes="0" failures="1" expectedFailures="0"/>
+      </Section>
+      <Section name="std::clog" filename="projects/<exe-name>/MessageTests.cpp" >
+        <OverallResults successes="0" failures="1" expectedFailures="0"/>
+      </Section>
+      <Section name="Interleaved writes to cerr and clog" filename="projects/<exe-name>/MessageTests.cpp" >
+        <OverallResults successes="0" failures="1" expectedFailures="0"/>
+      </Section>
+      <OverallResult success="false">
+        <StdErr>
+Write to std::cerr
+Write to std::clog
+Interleaved writes to error streams
+        </StdErr>
+      </OverallResult>
+    </TestCase>
     <TestCase name="Standard output from all sections is reported" tags="[.][hide][messages]" filename="projects/<exe-name>/MessageTests.cpp" >
       <Section name="one" filename="projects/<exe-name>/MessageTests.cpp" >
         <OverallResults successes="0" failures="1" expectedFailures="0"/>
@@ -10143,7 +10161,7 @@ spanner      <OverallResult success="true"/>
       </Section>
       <OverallResult success="true"/>
     </TestCase>
-    <OverallResults successes="859" failures="91" expectedFailures="21"/>
+    <OverallResults successes="859" failures="94" expectedFailures="21"/>
   </Group>
-  <OverallResults successes="859" failures="90" expectedFailures="21"/>
+  <OverallResults successes="859" failures="93" expectedFailures="21"/>
 </Catch>