Add compatibility test for mapReduce

[eerxuan]

Part of #25.

Category: core / aggregation / commands

Add compatibility test coverage for the mapReduce feature.

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

[danielfrankcom]

May be out of scope, deprecated in Mongo 5.x

[eerxuan]

Closing as descoped. Reason: Deprecated in 5.x.