Correct .gitignore in several places for kselftest outputs

[Erik Hollensbe <erik@xxxxxxxxxxxxx>]

Sincerely hope I did this right; first time contributor, learning the
patch workflow. I took the opportunity to fix two .gitignore files that
were leaving stale worktree/index outputs after running `make kselftest`
against recent mainline (76c057c84d286140c6c416c3b4ba832cd1d8984e).

Thanks for your time!