Always write test xml reports, and under dist/

[benjyw]

Today we always generate and collect the xml reports for each test run (for pytest, junit, scalatest) but we discard them unless the user specifies --test-xml-dir=foo/bar, in which case we write them under /foo/bar.

This is at odds with how we write, say, coverage data, where the user asks for a report with, e.g., --test-use-coverage --coverage-py-report=json and then we pick the directory under dist/, ensuring no naming collisions.

This issue proposes that:

A) We write these reports at a subdir of our choosing, under dist/.
B) We always write them, since we always gather them.

One might quibble with B). Perhaps we want an option to turn this off, but in that case I propose not getting too clever - a single boolean flag to turn it off globally will suffice.

[jsirois]

In the interest of knowing why we diverge, the original PR #9594.

[Eric-Arellano]

Both sound good to me, including a single global option to turn it off.

Do note that several users are using this current implementation in CI..so I recommend we respect the deprecation policy, even though that will be clunky to implement.

[benjyw]

In the interest of knowing why we diverge, the original PR #9594.

Yeah, I think back then we didn't have a good sense of what any of this should look like in v2.

[benjyw]

Closed in #14929