fstests: add a mmap test group

fstests: add a mmap test group

Add a mmap group to tag tests that exercise memory mapped reads/writes, so
that it's easy to filter and run tests that exercise map:

   ./check -g mmap

Very useful during development to quickly test changes to a filesystem's
mmap implementation.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>