test: robustify mnt_tracefs test

* make sure that tracefs is actually loaded, and try and access it after
  restore
* don't mount debugfs in /, leaving an ugly directory in /test
* clean up the debugfs mount point (with a hook, since the test can't
  unmount it due to perms)

travis-ci: success for test: robustify mnt_tracefs test
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>