Replace std::endl with \n and flush

In some places the `std::flush` was not added, as it was sufficiently
obvious that the flush semantics are not intended. There are likely
other places where the flush semantics aren't intended, but that
is a cleanup for later.

src/catch2/internal/catch_xmlwriter.cpp

@@ -329,7 +329,7 @@ namespace {
 
     void XmlWriter::newlineIfNecessary() {
         if( m_needsNewline ) {
-            m_os << std::endl;
+            m_os << '\n' << std::flush;
             m_needsNewline = false;
         }
     }