tr: refactor test_stdin_is_socket

[xtqqczze]

No description provided.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[xtqqczze]

CICD / Code Coverage (ubuntu-latest, unix) (pull_request) failure tracked by #11736

[xtqqczze]

These two forms are not equivalent due to drop timing:

let mut x = File::from(writer);
x.write_all(b"::").unwrap();

File::from(writer).write_all(b"::").unwrap();

[xtqqczze]

I would like to use filedescriptor::socketpair from the filedescriptor crate here, something like: xtqqczze@26a0afd