tests/int: add a "update cpu period with pod limit set" test

Add a test case for an issue fixed by the previous commit.

Unfortunately, this is somewhat complicated as there's no easy way to
create a transient unit, so a binary, sd-helper, had to be added. On top
of that, an ability to create a parent/pod cgroup is added to
helpers.bash, which might be useful for future integration tests.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.gitignore

@@ -2,6 +2,7 @@ vendor/pkg
 /runc
 /runc-*
 contrib/cmd/recvtty/recvtty
+contrib/cmd/sd-helper/sd-helper
 man/man8
 release
 Vagrantfile