Add compatibility test for $mergeObjects

[eerxuan]

Part of #28.

Category: core / operator / accumulators

Add compatibility test coverage for the mergeObjects feature.

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

[alinaliBQ]

/take

[github-actions]

@alinaliBQ Sorry, only org members or collaborators can self-assign issues.

[documentdb-triage-tool]

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