Run plugin tests against the debug engine

[gaaclarke]

Right now plugin tests run against the release version of the flutter engine. The release version of the engine has less verification for correctness. For example, in flutter/packages#6069 it was found that a platform channel in the google_maps_flutter plugin was being used from a background thread which is a violation that can introduce race conditions. It is silently ignored in release engines, but if plugin tests were run against the debug engine we'd catch those failures and potentially others.

It is my understanding that we save debug builds but they may not be readily served by the flutter tool.

This will solve the issue for our plugins, ideally this feature would be available to external plugin developers.

[stuartmorgan-g]

This would require a lot of custom tooling, but would be useful. Ideally the tooling would be in flutter so we wouldn't need to maintain a bunch of things in packages specifically (and so 3P developers could use it).