Make Testing Great Again

[pingpongboss]

Currently, testing Material Motion often involves defining one-shot Plans and Performers in test code. This is a ton of boilerplate.

The reason we have to do this is often because we want to verify a side-effect of the performer, but we cannot have a direct reference to it due to the loose Plan/Performer coupling.

To close this issue, we must have a framework in place for testing to be painless. No more creating Plan/Performer classes if possible.

[pingpongboss]

We might want to consider putting util classes like StepChoreographer into a separate package other than com.google.android.material.motion.runtime to visually separate it from the actual unit tests.

http://codereview.cc/D1868#38122

[pingpongboss]

All *Tests should be renamed *Test. Context: http://codereview.cc/D2174#inline-8392