Add compatibility test for multikey

[eerxuan]

Part of #27.

Category: core / indexes / types

Add compatibility test coverage for the multikey feature.

Note: a smoke test exists; comprehensive compatibility test coverage is needed.

[documentdb-triage-tool]

👋 @vic-tsang — this issue has been assigned for 14 days with no activity. Is it still in progress? If you've made progress, please leave a quick update. Otherwise it'll be flagged for re-assignment at the next triage meeting.