Apparently a locale can be available at MSW level, so that
wxLocale::IsAvailable() returns true, but not supported by the MSVC CRT,
so that constructing the corresponding wxXLocale fails, which resulted
in wxXLocale unit test failures.
Fix them by checking that wxXLocale can be constructed directly instead
of using wxLocale::IsAvailable() as a proxy.
This is not ideal and perhaps wxLocale::IsAvailable() should check that
the locale is supported in wxXLocale too, but should at least allow unit
tests to pass on AppVeyor for now.
01cd702ee3