btrfs: test for btrfs send when nested subvols/snapshots exist

btrfs: test for btrfs send when nested subvols/snapshots exist

Regression test for a btrfs incremental send issue where the difference
between the snapshots used by the incremental send consists of one of
these cases:

1) First snapshot has a directory with name X and in the second snapshot
   that directory doesn't exist anymore but a subvolume/snapshot with
   the same name (X) exists;

2) First snapshot has a subvolume/snapshot with name X and in the second
   snapshot that subvolume/snapshot doesn't exist anymore (might have been
   replaced by a directory with the same name or not).

This issue is fixed by the following linux kernel btrfs patches:

    Btrfs: send, don't error in the presence of subvols/snapshots
    Btrfs: set dead flag on the right root when destroying snapshot

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>