xfstests: renumber existing btrfs tests to start with 1

xfstests: renumber existing btrfs tests to start with 1

Current numbering is inheried from the single testsuite series. There
are only 6 btrfs-specific tests and it makes more sense to start adding
new ones at a more natural place than 300-something. There's no overlap
with the old and new numbers and I hope there' will be no confusion when
referencing them.

Signed-off-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>