display single contained exception in excgroups in test summary

[jakkdl]

fixes #12943

As ExceptionInfo still doesn't have proper support for exception groups this continues upon the hacky solution from #10209, modifying the reprcrash as well.

I find this solution ... incredibly hacky and ugly, and am not a fan that n==1 gets handled but not n==2. But conceptually it just gets really tricky to figure out what info to strip, and how to show that. If we're going with stripping structure & group messages we could extend this solution to something like [in ExceptionGroup]: ValueError("foo"), TypeError("bar").

EDIT: less hacky now that I moved the code to ExceptionInfo.exconly

[Zac-HD]

Simpler implementation:

def _get_single_subexc(
    eg: BaseExceptionGroup[BaseException],
) -> BaseException | None:
    if len(eg.exceptions) != 1:
        return None
    if isinstance(e := eg.exceptions[0], BaseExceptionGroup):
        return _get_single_subexc(e)
    return e

[Zac-HD]

Suggested change

	return f"[in {type(self.value).__name__}] {subexc!r}"
	return f"{subexc!r} [single exception in {type(self.value).__name__}]"

as discussed in the issue