Automatic merge from submit-queue

Use slices of items to clean up after tests

Fixes #27582.

We used to maintain a pointer variable for each process to kill after the
tests finish. @lavalamp suggested using a slice instead, which is a much
cleaner solution. This implements @lavalamp's suggestion and also extends
the idea to tracking directories that need to be removed after the tests finish.

This also means that we should no longer check for nil `killCmd`s inside
`func (k *killCmd) Kill() error {...}` (see #27582 and #27589). If a nil
`killCmd` makes it in there, something is bad elsewhere and we want to see
the nil pointer exception immediately.

Mentioning @timstclair and @euank wrt the original issue/PR.