jenkins: No parallelizm for group test

The head test is zdtm/lib/group, so it is impossible to run several
instances of it in parallel
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/jenkins/criu-groups.sh

@@ -4,4 +4,4 @@ source `dirname $0`/criu-lib.sh
 prep
 mount_tmpfs_to_dump
 ./test/zdtm.py group --max 32 -x maps04 -x cgroup || fail
-./test/zdtm.py --set groups run --all --report report --parallel 2 -f best || fail
+./test/zdtm.py --set groups run --all --report report -f best || fail