FIRQueryTests.mm: improve the test that resumes a query with existence filter to actually validate the existence filter

[dconeybe]

This PR improves the test testResumingAQueryShouldUseExistenceFilterToDetectDeletes() in FIRQueryTests.mm, which was added by the previous PR #10974, to also validate that the existence filter sent from Watch was correct. This was achieved by adding some test-only "hooks" into the SDK's internals that enable the test to "listen" for existence filter mismatches. This is a port of firebase/firebase-js-sdk#7149 and firebase/firebase-android-sdk#4813.

#no-changelog

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  Overall coverage changed from 88.06% (cd0456e) to 88.08% (e9be944) by +0.02%.

  Filename	Base (cd0456e)	Merge (e9be944)	Diff
  ordered_code.cc	94.39%	93.90%	-0.49%
  remote_event.cc	97.10%	97.12%	+0.02%
  task.cc	93.91%	96.52%	+2.61%
  testing_hooks.cc	?	100.00%	?
  write_stream.cc	94.37%	91.55%	-2.82%

Test Logs

• Base (cd0456e): https://github.com/firebase/firebase-ios-sdk/actions/runs/4873739388
• Merge (e9be944): https://github.com/firebase/firebase-ios-sdk/actions/runs/4873725601

1. https://storage.googleapis.com/firebase-sdk-metric-reports/uS4xDH7h2l.html

[google-oss-bot]

Size Report ¹

Affected Products

• FirebaseFirestore

  Type	Base (484ca20)	Merge (d8f9cb4)	Diff
  CocoaPods	?	-51.5 kB	? (?)

Test Logs

• Merge (d8f9cb4): https://github.com/firebase/firebase-ios-sdk/actions/runs/4865084488

1. https://storage.googleapis.com/firebase-sdk-metric-reports/5boRXb9qCb.html