Add unit test to verify all bundled themes have valid text domain declared

[markjaquith]

#26542 was embarrassing — we almost released Twenty Fourteen without a text domain declared for it. Let's set up a unit test to make sure this never happens again.

[nacin]

Would benefit from public exposure of WP_Theme::$_default_themes.

[nacin]

Or can just do a hardcoded array and also fail the test if WP_DEFAULT_THEME is not in the array, which would cause us to update it.

[nacin]

In 26858:

Add unit test to verify all bundled themes have valid text domain declared.

fixes #26566.