Add PDBs to test pods

[dguendisch]

Currently our test pods get terminated when cluster nodes are rolling (i.e. due to OS updates).
However we'd prefer to let the tests pass and roll the node only after the last test pod terminated on its own.

Hence some kind of PDBs are needed for the test pods to let mcm drain the nodes by waiting for PDBs to be fulfilled. Unfortunately we cannot specify .spec.maxUnavailable as the testpods are not supervised by a deployment/replicaset/... (see https://kubernetes.io/docs/tasks/run-application/configure-pdb/#arbitrary-controllers-and-selectors)
Hence we could only use .spec.minAvailable which is however pretty dynamic (as the number of currently active testpods changes all the time), so this would need to be applied continuously by some controller.