Add xunit timeout to hanging CancelledBuild test

[rainersigwald]

This was attempted in #5290 but we hit a hang in this test again this week (https://dev.azure.com/dnceng/public/_build/results?buildId=860511).

[Forgind]

Looks like we also hung in https://dev.azure.com/dnceng/public/_build/results?buildId=845339&view=results with windows full, but we weren't planning to merge it yet, so we didn't investigate. That means there it can happen on both full and core. I'm wondering if there are multiple places it can hang or if the timeout I added just doesn't work sometimes.

In any case, I'd approve but I think the Timeout parameter for Facts is being deprecated. See here. The most upvoted answer in that thread has an alternate method.

[rainersigwald]

In any case, I'd approve but I think the Timeout parameter for Facts is being deprecated. See here. The most upvoted answer in that thread has an alternate method.

As mentioned in the comments to that answer, this is no longer true. See xunit/xunit@dabc047. Our tests are single-threaded so we can use it.

[Forgind]

The later comment didn't sound terribly confident or include a link. It does look like that commit hasn't been reverted, though, so that's good.

Even though we aren't going for parallelizing it, async still would put it on a separate thread, right? Without having looked carefully at how the timeout is implemented, I'm wondering how/whether that works.

[rainersigwald]

Branch isn't quite open yet but I need to spin a new build so I'm going to optimistically merge this.