More robust cleanup of executors in test_kubernetes_executor (#28281)

As a follow up after #28047, this PR will make the test cleanup
more robust and resilient to any errors that might have caused
kubernetes_executors left behind.

wrapping start()/end() in try/finally will make the tests
completely resilient to cases where the asserts start to fail -
without those, any failure in tests would cause the same resource
leakage as we initially had when #28407 was iterated on.