Fix incorrect use of setlocale() in wxLocale::IsAvailable().

Fix incorrect use of setlocale() in wxLocale::IsAvailable().

The return value of setlocale() was used incorrectly in this code: it
represents the newly set locale and not the previously active one so we didn't
actually restore the original locale before.

Fix the code and check that we do actually restore the locale in a new unit
test for it.

See #13117.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67405 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775