Run MBConvTestCase::LibcTests() only for MSVC and not other Windows compilers.

Run MBConvTestCase::LibcTests() only for MSVC and not other Windows compilers.

This test fails for MinGW, probably because its CRT doesn't use the same
locale names as MSVC CRT. Just disable it for now, as it was already disabled
for non-MSW platforms.

Also use LocaleSetter class (extracted from CLocaleSetter) to change the
locale for this test duration only to avoid affecting any tests running after
it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65743 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775