Cleans up tests

Cleans up test_helper and tests by simplifying and improving the understandability of the test data we're using.

This also removes tests for destroying/restoring each type of relation (i.e., belongs_to v. has_many v. has_one v. has_one through), since there should not be a difference as long as a relation contains `dependent: :destroy`.

This also removes an old test for properly selecting columns, as this was pulled in for testing code in `#_set_destruction_state` which is no longer present.

Also adds a note of warning to the README about omitting `include DestroyedAt` on a through table with a parent and child relation that both include it.