Firestore: 'test_watch_query' systest flakes

[tseaver]

Similar to #6605. See: https://source.cloud.google.com/results/invocations/57661644-6fd2-4203-a515-4c3bf9edfcd5/targets/cloud-devrel%2Fclient-libraries%2Fgoogle-cloud-python%2Fpresubmit%2Ffirestore/log

_______________________________ test_watch_query _______________________________
client = <google.cloud.firestore_v1beta1.client.Client object at 0x7f61dc7705f8>
cleanup = <built-in method append of list object at 0x7f61d83f3688>
 def test_watch_query(client, cleanup):
 db = client
 doc_ref = db.collection(u"users").document(u"alovelace" + unique_resource_id())
 query_ref = db.collection(u"users").where("first", "==", u"Ada")
 # Initial setting
 doc_ref.set({u"first": u"Jane", u"last": u"Doe", u"born": 1900})
 sleep(1)
 # Setup listener
 def on_snapshot(docs, changes, read_time):
 on_snapshot.called_count += 1
 # A snapshot should return the same thing as if a query ran now.
 query_ran = db.collection(u"users").where("first", "==", u"Ada").get()
 assert len(docs) == len([i for i in query_ran])
 on_snapshot.called_count = 0
 query_ref.on_snapshot(on_snapshot)
 # Alter document
 doc_ref.set({u"first": u"Ada", u"last": u"Lovelace", u"born": 1815})
 for _ in range(10):
 if on_snapshot.called_count == 1:
 return
 sleep(1)
 if on_snapshot.called_count != 1:
 raise AssertionError(
 "Failed to get exactly one document change: count: "
> + str(on_snapshot.called_count)
 )
E AssertionError: Failed to get exactly one document change: count: 0
tests/system.py:795: AssertionError

[tseaver]

Haven't seen this failure since the New Year. Reopen if it reappears.