Fixes tests to use the correct context to enable async refactor

[marabooy]

Refactors tests and prepares code for #411

Moves all UI tests to use await async and removes the .Wait() call that could hide any async bugs or COM extension communication bugs. Fixes #412

Also introduces the use of Joinable task factory to switch to the background thread https://devblogs.microsoft.com/premier-developer/asynchronous-and-multithreaded-programming-within-vs-using-the-joinabletaskfactory/