Add tests that unmount a file system while it is busy. On a vnd

Add tests that unmount a file system while it is busy.  On a vnd
device "cp -r" and "rm -rf" are working in the background while
"umount -f" or "mount -ur" are executed.  Then "fsck -f" is used
to verify that the file system is clean.
Currently this triggers a bug quite reliably.  After the mount point
has been changed to read-only, the file system check shows that not
everything was flushed and synced properly.