Use standard timestamp format for debug messages in the tests

Don't use the current locale format which doesn't provide all the
information (e.g. never includes milliseconds) and explicitly set the
format providing it instead when showing wxLogTrace() messages.

tests/test.cpp

@@ -618,6 +618,8 @@ int TestApp::RunTests()
     // running the tests.
     if ( !wxGetEnv(wxASCII_STR("WXTRACE"), NULL) )
         wxLog::EnableLogging(false);
+    else
+        wxLog::SetTimestamp("%Y-%m-%d %H:%M:%S.%l");
 #endif
 
     // Cast is needed under MSW where Catch also provides an overload taking