fix: Without `@DirtiesContext` the app is not teared down correctly

fix: Without `@DirtiesContext` the app is not teared down correctly

* `StorageStrategy` depends on Spring to call the method
  `ChatBackendApplication#onExit()`, that is annotated with `@PreDestroy`.
* If this method is not called, the strategy is not applied, which leads
  to errors in the integration-tests.
* This happens, if all tests are run through Maven, because Spring
  recycles the application-context and only tears down the context after
  all tests have run.
* The addition of `@DirtiesContext` on those tests forces Spring to tear
  down the application completely after each so annotated test.
* Hence, the method is called as expected, which fixes the described
  errors.