KubernetesTestFixture doesn't implement the checkAndUpdateConfigMapFunction properly

FlinkKubeClient.checkAndUpdateConfigMap expects an error to be forwarded through the CompletableFuture instead of throwing a RuntimeException.

The actual implementation implements it accordingly in Fabric8FlinkKubeClient:313 where a CompletionException is thrown within the CompletableFuture's supplyAsync call resulting the future to fail.

KubernetesTestFixture doesn't make the returned future complete exceptionally but throws a CompletionException (see KubernetesTestFixture:172).

This results in inconsistent test behavior.