JUnit reporter uses only 3 decimal places when reporting durations

We used to use whatever precision we ended up having from C++'s
stdlib. However, some relatively popular tools, like Jenkins,
use Maven SureFire XML schema to validate JUnit test reports, and
Maven SureFire schema requires the duration to have at most 3
decimal places.

For compatibility, the JUnit reporter will now respect this
limitation.

Closes #2221

src/catch2/reporters/catch_reporter_junit.cpp

@@ -17,6 +17,7 @@
 #include <cassert>
 #include <ctime>
 #include <algorithm>
+#include <iomanip>
 
 namespace Catch {
 
@@ -54,6 +55,17 @@ namespace Catch {
             }
             return std::string();
         }
+
+        // Formats the duration in seconds to 3 decimal places.
+        // This is done because some genius defined Maven Surefire schema
+        // in a way that only accepts 3 decimal places, and tools like
+        // Jenkins use that schema for validation JUnit reporter output.
+        std::string formatDuration( double seconds ) {
+            ReusableStringStream rss;
+            rss << std::fixed << std::setprecision( 3 ) << seconds;
+            return rss.str();
+        }
+
     } // anonymous namespace
 
     JunitReporter::JunitReporter( ReporterConfig const& _config )
@@ -111,7 +123,7 @@ namespace Catch {
         if( m_config->showDurations() == ShowDurations::Never )
             xml.writeAttribute( "time"_sr, ""_sr );
         else
-            xml.writeAttribute( "time"_sr, suiteTime );
+            xml.writeAttribute( "time"_sr, formatDuration( suiteTime ) );
         xml.writeAttribute( "timestamp"_sr, getCurrentTimestamp() );
 
         // Write properties
@@ -178,7 +190,7 @@ namespace Catch {
                 xml.writeAttribute( "classname"_sr, className );
                 xml.writeAttribute( "name"_sr, name );
             }
-            xml.writeAttribute( "time"_sr, ::Catch::Detail::stringify( sectionNode.stats.durationInSeconds ) );
+            xml.writeAttribute( "time"_sr, formatDuration( sectionNode.stats.durationInSeconds ) );
             // This is not ideal, but it should be enough to mimic gtest's
             // junit output.
             // Ideally the JUnit reporter would also handle `skipTest`