Firestore: 'test_collection_add' flakes

[tseaver]

From https://source.cloud.google.com/results/invocations/7deedac3-8b60-447d-bb5e-74a4a6653e57/targets/cloud-devrel%2Fclient-libraries%2Fgoogle-cloud-python%2Fpresubmit%2Ffirestore/log:

_____________________________ test_collection_add ______________________________
client = <google.cloud.firestore_v1.client.Client object at 0x7fb9ca171ad0>
cleanup = <built-in method append of list object at 0x7fb9c7c0acb0>
    def test_collection_add(client, cleanup):
        collection1 = client.collection("collek")
        collection2 = client.collection("collek", "shun", "child")
        collection3 = client.collection("collek", "table", "child")
        explicit_doc_id = "hula" + unique_resource_id("-")
>       assert set(collection1.list_documents()) == set()
E       assert set([<google....fb9c7c06290>]) == set([])
E         Extra items in the left set:
E         <google.cloud.firestore_v1.document.DocumentReference object at 0x7fb9c7c06290>
E         Full diff:
E         - set([<google.cloud.firestore_v1.document.DocumentReference object at 0x7fb9c7c06290>])
E + set([])

This test should be using a unique / name-mangled collection ID, in order to prevent collision with CI running for other PRs.