Prior to this, in-place jail could not clean up nested directories
because it would try to remove the parents (which were necessarily
created first) before removing the parents, which would fail with a
"directory not empty" error.

The newly-added TestJailIntoCleansNestedDirs() would fail with this
error:

    Can't delete jail "/tmp/my-jail-1593477629". Can't delete directory
    in jail "/tmp/my-jail-1593477629/way": remove
    /tmp/my-jail-1593477629/way: directory not empty