btrfs: add basic qgroup testing

btrfs: add basic qgroup testing

We have no tests for testing qgroups, so we have no way of knowing
if our changes are breaking qgroups at all.  Get the ball rolling
with some basic functionality tests, these just make sure we can
enable quotas and do rescan and get sane values back, as well as
make sure the limiting stuff works properly.

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>