Clean up extraneous copies in Messages

This removes 109 allocations from running `tests/SelfTest`
(437057 -> 436948).

src/catch2/internal/catch_run_context.cpp

@@ -402,8 +402,8 @@ namespace Catch {
         m_messages.erase(std::remove(m_messages.begin(), m_messages.end(), message), m_messages.end());
     }
 
-    void RunContext::emplaceUnscopedMessage( MessageBuilder const& builder ) {
-        m_messageScopes.emplace_back( builder );
+    void RunContext::emplaceUnscopedMessage( MessageBuilder&& builder ) {
+        m_messageScopes.emplace_back( CATCH_MOVE(builder) );
     }
 
     std::string RunContext::getCurrentTestName() const {