Improve readability of relationships test (#1904)

* Really fix intermittent relationship test failures

* Unify the two JSON API relationship test files

I accidentally introduced the duplication when
merging
#1543 (comment)
and they've evolved separately since then.

They both have some value and need to be combined.

* Resolve duplicate tests from diverged tests files

* No longer test Association/Relationship interface directly

Author: bf4