Use `atty` crate in libtest instead of rolling our own

[camelid]

cc #80888
cc @jyn514

This issue has been assigned to @mattwilkinsonn via this comment.

[jyn514]

@camelid what happened to #42732 (comment) ?

[nagisa]

winapi has since been improved to no longer imply long build times at the very least.

[camelid]

But I think we should probably use atty since it's so widely used.

[artorias111]

Hey! Is this a good issue if I'm just getting used to contribute to various projects. Not really familiar with the rust codebase but would like to try if I can contribute.

[jyn514]

@artorias111 yes, this would be a good first issue I think. I'm not 100% convinced we should make the change, but the implementation is easy, just add atty as a dependency and use it in libtest: #80897 (comment)

[artorias111]

@jyn514 sure, I'll give it a try then

[diohabara]

@jyn514
Those participated do not seem to be working on this issue, and the related PR is also inactive.
So, can I assign this task to myself?

[diohabara]

I'll do it for now, but please let me know if you have any problems.

[diohabara]

@rustbot claim