Poll for PowerAppsTestEngine

Description

TestEngine needs to poll for the PowerAppsTestEngine global in the web player frame.

[ ] The code change is covered by unit tests. I have added tests that prove my fix is effective or that my feature works
[ ] I have performed end-to-end test locally.
[ ] New and existing unit tests pass locally with my changes
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation
[ ] My changes generate no new warnings
[ ] I used clear names for everything
[ ] I have performed a self-review of my own code