Add test for NoDocument assert

[schmidt-sebastian]

This PR adds a test for the assert that caused the Index-Free rollback on Android (b/141011925).

The customer got back to me (in person at the Firebase Summit) and their stacktrace is actually from MemoryPersistence. With MemoryPersistence it is actually much more likely to hit the scenario that we discussed as the possible culprit (in LocalStore.applyRemoteEvent):

if (existingDoc == null) {
  // This is the path that causes the assert. A failed limbo resolution added a 
  // NoDocument, but the limbo resolution fails when we have already deleted
  // the offending document from cache.
} else if (doc instanceof NoDocument) {
  // This is the good path 
}

With EagerGC, existingDoc is null if we already unlistened from the query that originally returned it when the Limbo resolution fails (or if we remove a mutation that depended on it).

The test hits the assert in the original code, but passes since we merged #2176

[mikelehen]

Oh niiiiiiiice. I knew we sent you to Fireconf for a reason!

Thanks for writing the test and adding the excellent comments in the test explaining what's going on. 👍