xfstests: btrfs/017: add a regression test for snapshot creation

xfstests: btrfs/017: add a regression test for snapshot creation

We had a regression where you couldn't snapshot a file system if you mounted it
ro and then remounted it rw.  This is a test that does just that to make sure we
don't have this problem again.  I ran the test without the fix and it blew up,
and then applied the fix and verified that it passed.  Thanks,

Reviewed-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>