test: Mark document snapshot hash as flaky (#699)

use the pytest.mark.xfail decorator

Author: Mariatta

packages/google-cloud-firestore/tests/unit/v1/test_base_document.py

@@ -271,6 +271,7 @@ def test_documentsnapshot___eq___same_reference_same_data():
     assert snapshot == other
 
 
+@pytest.mark.xfail(strict=False)
 def test_documentsnapshot___hash__():
     import datetime
     from proto.datetime_helpers import DatetimeWithNanoseconds