fix flaky abort retry error

tests/unit/test_session.py

@@ -1702,8 +1702,9 @@ def unit_of_work(txn, *args, **kw):
         def _time(_results=[1, 2, 4, 8]):
             return _results.pop(0)
 
-        with mock.patch("time.time", _time):
-            with mock.patch("time.sleep") as sleep_mock:
+        with mock.patch("time.time", _time), \
+             mock.patch("google.cloud.spanner_v1._helpers.random.random", return_value=0), \
+             mock.patch("time.sleep") as sleep_mock:
                 with self.assertRaises(Aborted):
                     session.run_in_transaction(unit_of_work, timeout_secs=8)