FileStream: enable automatic flushing

when a test executable is killed by a signal (e.g. when executed by
ctest with timeout), the reporter files are not flushed. this can lead
to incomplete (or empty) report files.
to avoid this we enable automatic flushing via `std::unitbuf`

compare #663

src/catch2/internal/catch_istream.cpp

@@ -78,6 +78,7 @@ namespace Detail {
             FileStream( std::string const& filename ) {
                 m_ofs.open( filename.c_str() );
                 CATCH_ENFORCE( !m_ofs.fail(), "Unable to open file: '" << filename << '\'' );
+                m_ofs << std::unitbuf;
             }
             ~FileStream() override = default;
         public: // IStream