Cgroup2: Cleanup procs in TestKill

[dcantah]

If the test fails before we get to calling manager.Kill we'll leak the 5 sleep processes, and its harmless to call this
at the end of the test also. This additionally removes the cmd.Process nil check as the comment in the stdlib states that
cmd.Process will be filled in if Start returns successfully.

[manugupt1]

Just a small comment: Should we kill all managers on cleanup using manager.Kill?

[dcantah]

@manugupt1 I did a Kill on anything that actually launched any processes, I'd say it's case by case BUT we could add a test helper in a follow up here to check the number of processes running -> Call Kill if there is any -> Always call delete

[manugupt1]

Oh okay! I was just wondering.

[dcantah]

I think it's a good idea! It's easy to forget to cleanup so if future authors see we always just call 'CleanupCgroup' or something similar the situation would be better I think

[fuweid]

LGTM

[dcantah]

@manugupt1 Oh woops, I just realized your comment for calling manager.Kill was on this PR and not #282. So the rationale here is if the process failed to join the cg we'd leak it. There's another test that adds a process to a cg here, TestMoveTo, which adds the current process to the CG.. which is annoying as we can't call Kill on the cg then, so I've changed that test to launch sleep infinity as well. I'll add the same Pdeathsig option on that PR.

[dcantah]

Resolved merge conflicts from 55c197e