firebase-ios-sdk

gRPC: add more unit tests to gRPC wrapper classes (#1924)

Add pretty exhaustive unit tests to `GrpcStream`, `GrpcStreamingReader`, and `GrpcUnaryCall`.

Also:
* when using `ForceFinish` to quickly do completions, verify that the completion that got off the queue has the expected type (this was previously just a comment). The difficulty with doing this in a more deterministic manner is that gRPC doesn't guarantee the order in which tags come off the completion queue. This mostly affects cases when there's a read and a write operation in progress at the same time. To work around this, `ForceFinish` now can also take a callback.
* use the ability to force-set a status on a completion to verify that the observer/callback receives the expected status;
* add the ability to force-set a received message on a completion to verify that the observer/callback receives the expected message;
* add comments about the usage of `ForceFinish`;
* Add `CreateNoOpConnectivityMonitor` helper function to `util` in tests.