RoundRobin Test flakiness fix (#177)

Added check for ensuring number of health endpoints are equal to REPLICA
befor proceeding
Run:
[ ]
[grpc/core/master/linux/psm-dualstack](https://source.cloud.google.com/results/invocations/25d32dcf-8413-4504-a1bf-ce7a31b13e20)

Author: pawbhard

tests/round_robin_test.py

@@ -63,7 +63,10 @@ def test_round_robin(self) -> None:
         with self.subTest("09_test_server_received_rpcs_from_test_client"):
             self.assertSuccessfulRpcs(test_client)
 
-        with self.subTest("10_round_robin"):
+        with self.subTest("10_test_healthy_endpoint_count"):
+            self.assertHealthyEndpointsCount(test_client, REPLICA_COUNT)
+
+        with self.subTest("11_round_robin"):
             num_rpcs = 100
             expected_rpcs_per_replica = num_rpcs / REPLICA_COUNT