firebase-ios-sdk

Have ListenerRegistration::Remove block until callbacks are complete (#5961)

* Add the ability to await a single expectation

* Await individual expectations in test helpers

This prevents tests from accidentally awaiting unrelated expectations
when using the helpers.

* Fix tests that implicitly relied on test helpers to await all

* Add a test that verifies ListenerRegistration::Remove blocks

* Block ListenerRegistration::Remove when there are outstanding callbacks.

* Test that remove does not block when called from the callback.