gh-131178: Use support.captured_stdout and support.captured_stderr

[donbarbos]

I replaced contextlib.redicrect_std{out,err} and support.captured_output("std{out,err}") with support.captured_std{out,err}

Inspired by #131287 (comment). I went beyond older PRs, I thought that in other tests this change is also relevant

• Issue: Add tests for command-line interface #131178

[picnixz]

Just change the recent CLI tests, not the rest. It produces too much churn otherwise. Serhiy was also against a global change

[donbarbos]

Just change the recent CLI tests, not the rest. It produces too much churn otherwise. Serhiy was also against a global change

Ok, and I found that test_unknown_flag in test_platform has assertStartsWith inside assertRaises, I hope you don't mind fixing it in the current PR

[picnixz]

Mmh, I really prefer separate commits as we could eventually not decide to change the capture_stdout() (I'll need to see the final changes and see if it's really a gain as there is also a backport question).

There won't be an opposition to fixing the test but there might be slight opposition in refactoring. I am travelling now so I suggest fixing the test first, pinging a core dev such as Adam Turner for the merge and wait for me to come back for the capture_stdout() change.

[inventshah]

See #136632 for a previous discussion about changing tests to use the support.captured_* helpers.

[serhiy-storchaka]

This is a code churn. Please leave it as is.