Enable cloud_firestore_mocks tests

[Kal-Elx]

Enable cloud_firestore_mocks tests when the library is updated to depend on the latest firebase libraries.
Revert 340b955 and update the library.

[Kal-Elx]

atn832/fake_cloud_firestore#149

[atn832]

I believe you were actually blocked by atn832/fake_cloud_firestore#154. You should be able to use the latest version at https://pub.dev/packages/cloud_firestore_mocks/changelog#081.