This patch removes mount.Exec entirely and instead uses the common
utility from k8s.io/utils/exec.
The fake exec implementation found in k8s.io/utils/exec differs a bit
than mount.Exec, with the ability to pre-script expected calls to
Command.CombinedOutput(), so tests that previously relied on a callback
mechanism to produce specific output have been updated to use that
mechanism.
367f879131