zdtm.py: don't umount in clean_tests_root
zdtm.py: don't umount in clean_tests_root

If we get EBUSY here, it means that we didn't umount in the test (i.e. we
didn't call fini() on some path). We should just do that instead.

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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