GQT runner doesn't try to remove cgroup mount dir
GQT runner doesn't try to remove cgroup mount dir

runner.DestroyAndStop(), which is called in most AfterEach GQTs, used to
recursively remove the runner's tmp dir (e.g.
/tmp/test-garden-$GinkgoParallelNode) and ignore errors.

Now, we recursively remove all children of that directory that is not a
cgroup root, and propagate errors.

[#146920227]

cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian