Firestore: 'test_watch_collection' systest flakes

[tseaver]

Similar to #6605, #6921.

From this Kokoro Firestore run:

____________________________ test_watch_collection _____________________________
client = <google.cloud.firestore_v1beta1.client.Client object at 0x7ff27e1c25d0>
cleanup = <built-in method append of list object at 0x7ff27e1ac878>
 def test_watch_collection(client, cleanup):
 db = client
 doc_ref = db.collection(u"users").document(u"alovelace" + unique_resource_id())
 collection_ref = db.collection(u"users")
 # Initial setting
 doc_ref.set({u"first": u"Jane", u"last": u"Doe", u"born": 1900})
 # Setup listener
 def on_snapshot(docs, changes, read_time):
 on_snapshot.called_count += 1
 for doc in [doc for doc in docs if doc.id == doc_ref.id]:
 on_snapshot.born = doc.get("born")
 on_snapshot.called_count = 0
 on_snapshot.born = 0
 collection_ref.on_snapshot(on_snapshot)
 # delay here so initial on_snapshot occurs and isn't combined with set
 sleep(1)
 doc_ref.set({u"first": u"Ada", u"last": u"Lovelace", u"born": 1815})
 for _ in range(10):
 if on_snapshot.born == 1815:
 break
 sleep(1)
 if on_snapshot.born != 1815:
 raise AssertionError(
> "Expected the last document update to update born: " + str(on_snapshot.born)
 )
E AssertionError: Expected the last document update to update born: 1900