Add tests for get_date_from_gmt() and get_gmt_from_date().

These tests demonstrate that get_date_from_gmt() is ignorant of DST settings.

props scholesmafia.
see #20328.



git-svn-id: https://unit-tests.svn.wordpress.org/trunk@1233 4a5a2adb-35eb-4268-9a1b-fe04a4134342