Correctly restore the old locale in wxXLocale functions.

Correctly restore the old locale in wxXLocale functions.

In non-wxHAS_XLOCALE_SUPPORT case we didn't restore the original locale
correctly in wxStrtoxxx_l() functions as the return value of wxSetlocale() was
incorrectly assumed to be the old locale instead of the new one.

Fix this and also replace the macros used by the old code with a small helper
class, this simplifies the code and is less ugly.

Finally add a unit test which failed before these changes when the program ran
in any non-C locale but passes now.

Closes #13117.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67406 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775