xUnit is no longer able to run tests on UWP. MSTest supports this in
Visual Studio 2022, so we are moving back to MSTest. (Rx used to used
MSTest before, so this was a relatively straightforward change.)

Apparently supplying a SynchronizationContext isn't enough to prevent the EventLoop_ScheduleActionNested and EventLoop_ScheduleActionDue tests from hanging on the build agent.

We'll need to fix this properly, so #1885 will track this

Fix race condition in CreateAsyncTest

Make TaskLikeSupportTests run both with and without SynchronizationContext

Revert the -v n option for test - hoping we now won't need it, now that we've configured test timeouts

We had --filter "SkipCI!=true" but according to the docs, what we want is --filter "TestCategory!=SkipCI"

We're still using xUnit's assertions. And we're also still using xUnit in the test project that verifies that the API surface area didn't change.

The newer API generator libraries have fixed some bugs - the 'verified' versions of these files used to have some missing type arguments. Any params argument using a generic type would just have an empty <>. Also, handling of the System.AttributeTargets looks like it used to be broken. The various .verified.cs files representing the expected API surface area have been modified so that they no longer have to work around these issues.

Unfortunately xunit.assert.source appears to have a bug causing it to inject a public type, so we've had to work around that.

Disable the handful of these that can't be expected to work on the current UWP implementation