Add some more tests exercising bind mounts on the same device which
should not be affected by -mount or -xdev, and tests with a loopback
file system and a bind mount from there.
* tests/find/mount-vs-xdev-bind.sh: Add test.
* tests/find/mount-vs-xdev-other-fs.sh: Likewise.
* tests/local.mk (all_root_tests): Reference them.
