kselftests for memory.oom.group

Hi Shuah,

I wrote some tests for the new memory.oom.group feature in cgroups 2.

In the process, I discovered a few small bugs in the cgroups tests, which I have fixed as well in a separate commit.

This is my first ever patch to Linux, so let me know if you see any issues or improvements that can be made.

Thanks for all your amazing work on Linux! -Jay