interop-test, core: retryTest is flaky

```
> Task :grpc-interop-testing:test

io.grpc.testing.integration.RetryTest > statsRecorded FAILED
    java.lang.NullPointerException
        at io.grpc.testing.integration.RetryTest.assertRpcStatusRecorded(RetryTest.java:248)
        at io.grpc.testing.integration.RetryTest.statsRecorded(RetryTest.java:354)
```


cc. @larry-safran 