Test Reliability: Test infra should handle cases where test app is left in minimized state

[kmahone]

This CI build failed due to test failures:
https://dev.azure.com/ms/microsoft-ui-xaml/_build/results?buildId=33255&view=logs

The first failing test is:
AcrylicBrushTests.HideAndShowWindow
after that, the rest of the tests in that test class also fail. And they all also fail on re-run.

This issue is not about the specific test failure, but rather the test infra reliability issue that it exposes.

For some reason, this test crashed in the middle of execution. As a result of this, the machine was left in a configuration that it was unable to recover from:
The test app was left running, but minimized.

Issues:

1. When we attempt to Close the app as part of cleanup via Application.Close, we fail to close the app due to not being able to find the appropriate UIA element ("Failed to find close app invoker:"), since the application is minimized. So after cleanup, the app is left running.
2. When the next test in the test class attempts to run, it attempts to launch the application (Application.Initialize). The app is already running, so it does not attempt to launch it. But it cannot find its CoreWindow (since the app is minimized), so it fails.
3. When the te.exe run is complete, we attempt to re-run the failed tests. Even though we are starting with a new clean process, etc. the test app is still running and minimized, so we hit the same issue in NavigationView TogglePaneTopPadding issue #2 again for all the re-runs.

Fixes:

1. As part of Application.Close we should try to terminate the process if we cannot close it via UIA.
2. As part of Application.Initialize, we should try to maximize the top level window, before trying to find the CoreWindow that is a child of it,
3. In the runtests.cmd script, we should ensure that we terminate any left over test processes before we do any re-runs.