Break Down JSONSerializer#serializeBelongsTo Test

There were multiple assertions in "serializeBelongsTo" test. Break it
into two to make things easier to understand.
Revert "remove unneeded and misleading "async" test helpers"

This reverts commit f34d87a0e8d4ed94a30b86ef07b3cf476748c3f6.

Failures on beta tests, will invetigate shortly.
Merge pull request #1734 from jherdman/tidy-up-json-serializer-test

Break Down JSONSerializer#serializeBelongsTo Test