Add stream flushes after reporters print out the test run metadata.

This ensures that the test run metadata are printed even if the
process dies right after.

Closes #2964

src/catch2/reporters/catch_reporter_compact.cpp

@@ -215,7 +215,8 @@ private:
                          << m_config->testSpec()
                          << '\n';
             }
-            m_stream << "RNG seed: " << getSeed() << '\n';
+            m_stream << "RNG seed: " << getSeed() << '\n'
+                     << std::flush;
         }
 
         void CompactReporter::assertionEnded( AssertionStats const& _assertionStats ) {

src/catch2/reporters/catch_reporter_console.cpp

@@ -529,7 +529,8 @@ void ConsoleReporter::testRunStarting(TestRunInfo const& _testRunInfo) {
         m_stream << m_colour->guardColour( Colour::BrightYellow ) << "Filters: "
                  << m_config->testSpec() << '\n';
     }
-    m_stream << "Randomness seeded to: " << getSeed() << '\n';
+    m_stream << "Randomness seeded to: " << getSeed() << '\n'
+             << std::flush;
 }
 
 void ConsoleReporter::lazyPrint() {

src/catch2/reporters/catch_reporter_tap.cpp

@@ -196,7 +196,8 @@ namespace Catch {
         if ( m_config->testSpec().hasFilters() ) {
             m_stream << "# filters: " << m_config->testSpec() << '\n';
         }
-        m_stream << "# rng-seed: " << m_config->rngSeed() << '\n';
+        m_stream << "# rng-seed: " << m_config->rngSeed() << '\n'
+                 << std::flush;
     }
 
     void TAPReporter::noMatchingTestCases( StringRef unmatchedSpec ) {