While working on SERVER-85376 I noticed a lack of jstests for accumulators. Inside of jstests/aggregation/accumulators there is no merge_objects.js for example, and while it is inside of jstests/aggregation/expressions this is different expected behavior. There are some other files that do use $mergeObjects as an accumulator, e.g. jstests/aggregation/sources/setWindowFields/parse.js uses it inside of $setWindowFields, but the examples and the tested behavior is limited.