Minor bug fixes, test modifications
- Fixed some validation bugs
- Added global $post to the setUp() method in tests, making it
  available to all the tests
- Removed test `testMissingIDExpectsBoxNotValidate` from L885
  Since 5df53f8 this tests is unnecessary. The validation method
  used to check the post ID through the request's POST data, we
  now rely on the $post object cached by WordPress. This is more
  robust and more reliable. See PR #5