Because we run our golang integration tests "in tree" (due to
the need to be under $GOPATH and have a vendor folder around), the
"command-output" folder was getting left behind, dirtying the worktree
after building.
This change does two things:
1. On a succecssful run, remove the folder.
2. Ignore the folder via .gitignore (this way if a test fails and you
do `git add .` you don't end up commiting this folder).
2669f65b39