Improve test coverage and add back "Allow voting" display option

MR 32 does the following:

1. Improves the test coverage, much of this was based on #3131954-26: Port tests to 8.x-1.x → but which were never included as part of that issue.
2. Test reworking includes updating FivestarTest::testViewerNonRating() which was testing a non existent disabled option which was never ported from Drupal 7. This is what I've added as allow_vote given that disabled is a pretty overloaded term.
3. Also worth noting that FivestarTest::testViewerNonRating() was added to in #3190556: Illegal choice 0 in field_fivestar element for fivestar widget → , however that additionis now covered elsewhere in FivestarTest. Aside: At a glance I can't say I understand what was achieved in #3190556 ¯\_(ツ)_/¯
4. Note I've pulled in some of the test changes made to FivestarTestTrait in #3201584-22: StarsFormatter - Add back user, smart and dual option for star display and text → as they were also needed here.

Would be nice to get this test coverage reviewed and in, at which point I'm happy to circle through other open issues and update them as needed.

Bumping so I don't miss reviewing this.