Follow up tasks for the move to xunit v3

[radical]

Follow up work for #8403:

• Re-enable xunit1051 warning, and fix the instances (xUnit1051: Calls to methods which accept CancellationToken should use TestContext.Current.CancellationToken)
• Re-enable xunit2031 also. (xUnit2031: Do not use Where clause with Assert.Single)
• Explore what bits can/should be updated in the repo to use v3 well

cc @Youssef1313 @RussKie

[joperezr]

Where you planning on taking this one for 9.3 @radical ?

[radical]

Where you planning on taking this one for 9.3 @radical ?

I wasn't. I had opened this to track follow up work required from #8403 as discussed, so it doesn't get missed. cc @RussKie