nohup: write unit tests to improve coverage

[sylvestre]

We almost don't have tests for nohup. We should have more:
https://codecov.io/gh/uutils/coreutils/tree/master/src/uu/nohup/src

Should be added here:
https://github.com/uutils/coreutils/blob/master/tests/by-util/test_nohup.rs

[sivachandran]

The implementation gets exercised only if stdin, stdout and stderr are connected with interactive terminals. I can't think of a straightforward way to do this from tests.

I'm thinking of using pty crate to fork a child with pseudo-terminal(pty) and invoke the implementation. I haven't tried this but it looks doable.

@sylvestre Let me know if this approach is fine or if there is a better approach.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[fabracht]

This is the oldest beginner friendly issue. I would like to tackle it. Would this be ok? This is my first time on opensource, so I would appreciate some tips along the process, if you guys don't mind. I'll start looking at the code today to understand a little bit better.

[sylvestre]

sure, please give it a try :)