selftests: cgroup: Make test_pids backwards compatible

selftests: cgroup: Make test_pids backwards compatible

The predicates in test expect event counting from 73e75e6fc352b
("cgroup/pids: Separate semantics of pids.events related to pids.max")
and the test would fail on older kernels. We want to have one version of
tests for all, so detect the feature and skip the test on old kernels.
(The test could even switch to check v1 semantics based on the flag but
keep it simple for now.)

Fixes: 9f34c566027b6 ("selftests: cgroup: Add basic tests for pids controller")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Tested-by: Sebastian Chlad <sebastian.chlad@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>