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

include/internal/catch_run_context.hpp

@@ -22,6 +22,7 @@
 #include "catch_result_builder.h"
 #include "catch_fatal_condition.hpp"
 
+
 #include <set>
 #include <string>
 
@@ -50,6 +51,29 @@ namespace Catch {
         std::string& m_targetString;
     };
 
+    // StdErr has two constituent streams in C++, std::cerr and std::clog
+    // This means that we need to redirect 2 streams into 1 to keep proper
+    // order of writes and cannot use StreamRedirect on its own
+    class StdErrRedirect {
+    public:
+        StdErrRedirect(std::string& targetString)
+        :m_cerrBuf( cerr().rdbuf() ), m_clogBuf(clog().rdbuf()),
+        m_targetString(targetString){
+            cerr().rdbuf(m_oss.rdbuf());
+            clog().rdbuf(m_oss.rdbuf());
+        }
+        ~StdErrRedirect() {
+            m_targetString += m_oss.str();
+            cerr().rdbuf(m_cerrBuf);
+            clog().rdbuf(m_clogBuf);
+        }
+    private:
+        std::streambuf* m_cerrBuf;
+        std::streambuf* m_clogBuf;
+        std::ostringstream m_oss;
+        std::string& m_targetString;
+    };
+
     ///////////////////////////////////////////////////////////////////////////
 
     class RunContext : public IResultCapture, public IRunner {
@@ -305,7 +329,7 @@ namespace Catch {
                 timer.start();
                 if( m_reporter->getPreferences().shouldRedirectStdOut ) {
                     StreamRedirect coutRedir( Catch::cout(), redirectedCout );
-                    StreamRedirect cerrRedir( Catch::cerr(), redirectedCerr );
+                    StdErrRedirect errRedir( redirectedCerr );
                     invokeActiveTestCase();
                 }
                 else {

include/internal/catch_stream.h

@@ -21,6 +21,7 @@ namespace Catch {
 
     std::ostream& cout();
     std::ostream& cerr();
+    std::ostream& clog();
 
 
     struct IStream {

include/internal/catch_stream.hpp

@@ -103,6 +103,9 @@ namespace Catch {
     std::ostream& cerr() {
         return std::cerr;
     }
+    std::ostream& clog() {
+        return std::clog;
+    }
 #endif
 }