check: Make -g and -x option meet description in manual

check: Make -g and -x option meet description in manual

In manual:
 # ./check --help
 ...
 -g group[,group...] include tests from these groups
 -x group[,group...] exclude tests from these groups
 ...

Above format is not supported, because ',' is not parsed in code.
This patch fixed it.

Before patch:
 # ./check -g scrub,replace
 Group "scrub,replace" is empty or not defined?

After patch:
 # ./check -g scrub,replace
 FSTYP         -- btrfs
 PLATFORM      -- Linux/x86_64 kerneldev 4.2.0-rc2_HEAD_c65b99f046843d2455aa231747b5a07a999a9f3d_+
 MKFS_OPTIONS  -- /dev/vdd
 MOUNT_OPTIONS -- /dev/vdd /var/ltf/tester/scratch_mnt
 ...(right result)...

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>