Flaky-test: TopicsTest.cleanup

https://github.com/apache/pulsar/runs/7576535389?check_suite_focus=true

```
Error: Tests run: 43, Failures: 14, Errors: 0, Skipped: 28, Time elapsed: 9.942 s <<< FAILURE! - in org.apache.pulsar.broker.admin.TopicsTest
Error: cleanup(org.apache.pulsar.broker.admin.TopicsTest) Time elapsed: 2.18 s <<< FAILURE!
java.lang.ClassCastException: class org.apache.pulsar.broker.namespace.NamespaceService cannot be cast to class org.apache.pulsar.broker.ServiceConfiguration (org.apache.pulsar.broker.namespace.NamespaceService and org.apache.pulsar.broker.ServiceConfiguration are in unnamed module of loader 'app')
at org.apache.pulsar.broker.PulsarService.getConfiguration(PulsarService.java:620)
at org.apache.pulsar.broker.auth.MockedPulsarServiceBaseTest.stopBroker(MockedPulsarServiceBaseTest.java:299)
at org.apache.pulsar.broker.auth.MockedPulsarServiceBaseTest.internalCleanup(MockedPulsarServiceBaseTest.java:239)
```

Start by running or inspecting org.apache.pulsar.broker.admin.TopicsTest, especially cleanup, and follow the stack trace through MockedPulsarServiceBaseTest.stopBroker and PulsarService.getConfiguration. Determine why cleanup produces the ClassCastException and confirm that TopicsTest cleanup completes without the reported failure.

Written by the indexing model from the issue text.