[libc++] Try re-enabling long-disabled locale tests on Apple platforms

These two locale tests are disabled because they were said to "pass in
an uncontrolled manner on Apple platforms". This commit re-enables them
to see what that means, and whether that is still relevant on the
platforms we test.

Before this commit, the tests were either XFAILed or UNSUPPORTED on
Apple and Linux, which is pretty much all the systems we support. If
the tests truly don't work anywhere, they should be removed instead.