tests: Ensure that mountpoints are unmounted before exiting

tests: Ensure that mountpoints are unmounted before exiting

When all tests complete successfully, filesystems mounted by grub-fs-tester
will be unmounted before exiting. However, on certain test failures the
tester will exit with a failure code and not unmount previously mounted
filesystems. Now keep track of mounts and umounts and run an exit handler
on exit or process interruption that will umount all mounts that haven't
already been unmounted.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>