implement #12231 new flag --xfail-tb

[okken]

This implements issue #12231, making xfail tracebacks optional.
Introduces a new flag --xfail-tb, that acts just like --tb, but just for xfail tracebacks.

With this feature in place, turning off xfail tracebacks is as simple as --xfail-tb=no, and even works when -rx or -ra are present.

This is a first pass at the PR.

• One test has been modified.

Still todo:

• Modify documentation for new flag
• Create changelog entry
• I think that's it ??

[nicoddemus]

Thanks @okken for tackling this so quickly!

As you mention indeed we still need tests and docs for this, but also I would like to see some other specific tests:

1. a normal failure and an xfail failure, with tb=line and xfail-tb=auto, and check the output.
2. I suppose just changing --xfail-tb default to no will require update some test(s), but if not, we need to check that the default does not produce any xfail tracebacks.

[okken]

I've written a rather verbose changelog entry. Feel free to shorten, tighten, completely rewrite, make suggestions, etc.

[okken]

@nicoddemus I think I've done all necessary changes

[okken]

I didn't quite do the tests as requested. by "a normal failure and an xfail failure, with tb=line and xfail-tb=auto, and check the output."

1. I didn't see the "a normal failure and" part until just now. I can add that.
2. None of the --tb tests use auto, but use short. So I did that.

Right now I've got a -rx test, an -rx --xfail-tb=short, and an -rx --xfail-tb=line test.

I think that's sufficient.
But if you'd like me to add the "and normal failure" part to the test, I can do that also.

[okken]

Converting to draft, because I'm still working on it, and thinking about behavior. see #12231 for discussion and questions

[okken]

Accidentally deleted my fork, along with this branch.
I've recreated the PR as #12280