new: only allow documented test group names

new: only allow documented test group names

Now that we require all group names to be listed in doc/group-names.txt,
we can use that (instead of running mkgroupfile) to check if the group
name(s) supplied by the user actually exist.  This has the secondary
effect of being a second nudge towards keeping the description of groups
up to date.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>