Currently the memory tracking unit tests test for non-zero memory usage to make sure that we are always accounting for memory usage in different execution paths, but also to give flexibility in adding more memory accounting later on

https://github.com/10gen/mongo/blob/a3fabcd884ce76dfae45c38af5c5bbe2dba19cda/src/mongo/dbtests/query_stage_sort.cpp#L202-L203 

We could consider adding asserts for an exact amount of bytes used, but that may be flakey on evergreen depending on the machine type. As a compromise, we could consider adding asserts that the memory reported should be in a certain range: e.g. if we are here, the bytes used should be non-zero but also less than X (X could be something like the internal memory limit of the stage, or 2x the size of the raw data used in the test).