Add tiny fix for system monitor and big test of jupyter

With the new test, tiny errors, like we had in the last images are not so likely to slip through. Though in the case of this fix, the expectation that the number of CPUs is an integer was just wrong. So for this issue the test maybe would not have helped, except we set more environment variables for testing that K8s and KubeSpawner or so set. So the new test can help to prevent these issues in future, but a bit of work has to be done to get the test environment closer to the production environment.