As of flutter/engine#50414, scenario_app on Android is now running on CI and is enforced.

As a next step, I'd like to run the same set of tests with --enable-impeller. A few caveats/points:

1. Based on my chat with @dnfield, we'll do Vulkan and OpenGLES.
2. iOS is already running on Impeller, I don't plan to make changes to have it run the Skia backend as well.
3. We'll have to deal with cases where there is an expected failure/crash, as not everything is implemented in every backend yet.

A few complications to solve (I just don't know how to do it yet):

• How do I start scenario_app with --enable-impeller?

  In "real" land, I'd pass the argument to FlutterMain.ensureInitializationComplete.

• How do I "shard" the Skia gold tests so that different backends use different keys?

  I'm guessing we have prior-art elsewhere in Impeller, I just don't know about it yet.

• How do I allow certain tests to fail (I want to avoid a "status file"-type messes)

  Maybe this is built-into JUnit and I just need to look a little deeper?

There might be other things I need to tackle, but that's what I know so far.

/cc @gaaclarke @dnfield @jonahwilliams @johnmccutchan for FYI.