implement #12231 new flag --xfail-tb

[okken]

This implements issue #12231, making xfail tracebacks optional.
Add --xfail-tb flag, which turns traceback output for XFAIL results.

• If the --xfail-tb flag is not sent, tracebacks for XFAIL results are NOT shown.
• The style of traceback for XFAIL is set with --tb, and can be auto|long|short|line|native|no.
• Note: Even if you have --xfail-tb set, you won't see them if --tb=no.

Some history:

• This is a behavior break, but brings default behavior back to pre-8.0.0 behavior.
• With this change, default -rx/ -ra behavior is identical to pre-8.0 with respect to xfail tracebacks.
• With pytest 8.0, -rx or -ra would not only turn on summary reports for xfail, but also report the tracebacks for xfail results.
• This caused issues with some projects that utilize xfail, but don't want to see all of the xfail tracebacks.
• This change detaches xfail tracebacks from -rx, and now we turn on xfail tracebacks with --xfail-tb.

[nicoddemus]

Looks great, thanks @okken!

I have left some minor requests, please take a look.

[okken]

Looks great, thanks @okken!

I have left some minor requests, please take a look.

Thanks @nicoddemus. I like all the suggestions. I've accepted all of them.

[okken]

Let me know if there's anything else needed.

[nicoddemus]

Thanks, looks great!

I will let it open for a few more days to give others a chance of reviewing it too. 👍

[nicoddemus]

(Note: we should squash/merge this)

[bluetech]

LGTM, thanks!

[okken]

@nicoddemus Squash and merge ok now? Or wanna wait a bit more?

[nicoddemus]

Thanks @okken!