Add support for xunit v3

[mattleibow]

Time to move on and up... to attachments.

We should make a new set of projects that target xunit.v3 and allow the user to pick their version.

[copilot-swe-agent]

@mattleibow I hit the following errors while trying to work on issue #1425.

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After resolving these issues, you can ask me to try again by unassigning and then reassigning the issue to me again.