firebase-ios-sdk

Merge remote_test_util and remote_testing libraries (#5254)

These two libraries duplicate each other in purpose:

  * firebase_firestore_remote_test_util
  * firebase_firestore_remote_testing

Additionally, several of the testing utilities in remote_test_util are
really only applicable to tests in the remote library anyway. These have
been put directly into firebase_firestore_remote_test.