btrfs-progs: tests: correctly receive clones to mounted subvol

btrfs-progs: tests: correctly receive clones to mounted subvol

clone needs to resolve the paths of the involved subvolumes in the target
fs from their UUIDs. When doing so it might need to strip the prefix
that is mounted as the root of the fs from those paths.

It didn't do so correctly when processing the source of "clone" commands

This is a regression test for
    btrfs-progs: receive: handle root subvol path in clone

Signed-off-by: Benedikt Morbach <benedikt.morbach@googlemail.com>
[ copied the fstests version, will be updated later ]
Signed-off-by: David Sterba <dsterba@suse.com>