Test the adaptive scheduler

With FLINK-21075, we introduced a new scheduler type which first waits for resources before deciding on the actual parallelism. This allows to continue executing a job even if the cluster loses a TaskManager permanently. We should test that this feature works as described by its documentation [1] (w/o using the reactive mode).

[1] https://ci.apache.org/projects/flink/flink-docs-master/docs/deployment/elastic_scaling/#adaptive-scheduler