Address flaky UI framework tests

[bartdesmet]

These WinForms and WPF tests go all the way back to a private test infrastructure (prior to OSS) and later MsTest, and never were hardened against concurrent execution of test cases; they were always run sequentially. This has led to flakiness at some point later, maybe during the migration to Xunit, and it seems nobody ever addressed this.

This PR hardens things in a few ways:

• Ensure only one test for WinForms can run at a given point in time, to avoid multiple running UI threads. Same for WPF.
• Make tests hygienic, i.e. guarantee clean up of the thread they've spawn, and exiting the message loop (e.g. dispatcher).
• Avoid calling Application.Exit on a different thread.
• Simplify the way to write test code.

After this goes in, we can likely re-enable some tests that were disabled, possibly because folks observed flakiness (e.g. ObserveOn tests getting stuck indefinitely because another test case ran Application.Exit and killed the message loop from underneath the test).