tools: make sure that test groups are described in the documentation

tools: make sure that test groups are described in the documentation

Create a file to document the purpose of each test group that is
currently defined in fstests, and change mkgroupfile to check that every
group mentioned in the tests is also mentioned in the documentation.

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