Add more tests cases to test suite for $push, $pull, and $inc.
Add more tests cases to test suite for $push, $pull, and $inc.

drorasaf/mongomock,akun/mongomock,chartbeat-labs/mongomock,julianhille/mongomock,magaman384/mongomock,vmalloc/mongomock,StarfishStorage/mongomock,marcinbarczynski/mongomock,mdomke/mongomock