results-2021-03-22: Close STDIN to prevent git from prompting for username/password (#46)

Also, make the test with the non-existing repo more robust: GitHub doesn't allow
`!` in repo names, so trying to clone `DOES_NOT_EXIST!!!.jl` would fail
immediately anyway.  Instead we want to test that trying to clone a repo with a
valid name doesn't prompt for username/password.