CDRIVER-4350 unskip mock tests and remove unnecessary skips (#1437)

* unskip mock tests referenced in CDRIVER-4350

* unskip `entity-client-storeEventsAsEntities`

As of https://github.com/mongodb/mongo-c-driver/pull/1220 the `storeEventsAsEntities` operation is supported in the test runner

* remove `/unified/entity-client-cmap-events` from skip-tests.txt

Test is already skipped in runner.c

* remove `/unified/entity-client-storeEventsAsEntities` from skip-tests.txt

Test is already skipped in runner.c

* remove `/unified/assertNumberConnectionsCheckedOut` from skip-tests.txt

Test is already skipped in runner.c