xfstests: btrfs tests for basic informational commands

xfstests: btrfs tests for basic informational commands

Run basic btrfs information commands in various ways, performing
sanity tests of: filesystem show, label, sync, and device stats
(sync is included just because it's simple).  These are mostly
just smoke tests, although for example show by label & UUID
should verify that the correct fs was shown.

This also adds quite a few new filters to accommodate the output
of the new commands.

Cc: linux-btrfs@vger.kernel.org
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>