Add compatibility test for $max (update field)

[eerxuan]

Part of #38.

Category: core / operator / update / fields

Add compatibility test coverage for the max feature.

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

[vic-tsang]

/take

[github-actions]

Assigned to @vic-tsang!

[documentdb-triage-tool]

👋 @imforster, @vic-tsang — this issue has been assigned for 15 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.