Correctly restore the originally used C locale in wxLocale dtor.

Save the original locale used before we changed it instead of "restoring" the
same locate that this wxLocale object was using.

Add a unit test to verify that this does work as expected.

Closes #14873.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74426 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775