Fix totally broken LocaleSetter class in the test suite.

Fix totally broken LocaleSetter class in the test suite.

It was based on a completely erroneous assumption that setlocale() returns the
locale that had been previously active when it actually returns the newly set
locale.

This fixes unit test failures in StringTestCase under OS X, as the locale
wasn't correctly restored by DateTimeTestCase that used this class.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74522 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775