Check if WP_DEFAULT_THEME starts with twenty before asserting it is in default theme array

This fixes an issue that if you change WP_DEFAULT_THEME and run core unit tests, the tests fail since your theme isn't one of the hard coded lists of default themes. We need to keep this test to make sure that we update the array of default themes for use in other tests.

If we ever change the naming convention for default themes, this will need to be updated.

props nacin for initial idea
fixes #29925



git-svn-id: https://develop.svn.wordpress.org/trunk@29946 602fd350-edb4-49c9-b593-d223f7449a82