Fix VarArgTestCase compilation when type traits are unavailable.

Fix VarArgTestCase compilation when type traits are unavailable.

When type traits are unavailable we can't check whether a type can be passed
to a vararg function but we still need to pass a copyable object to
wxString::Format() for the code to compile, even if we just want to check that
it will fail with the assert at run-time.

Closes #13118.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67398 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775