Formalize how to launch chrome and chromedriver in tests #187756

It's not entirely clear how to run chrome and chromedriver in an engine or framework test.

Chrome and chromedriver directories are both appended to the PATH:
https://cs.opensource.google/flutter/recipes/+/main:recipe_modules/flutter_deps/api.py;l=265-266;drc=de11b76f70be6b53ab5e49bdfda959c42aaf0aa3

The Chrome executable also gets set to an environment variable so it is easier to launch from tests:
https://cs.opensource.google/flutter/recipes/+/main:recipe_modules/flutter_deps/api.py;l=283-285;drc=de11b76f70be6b53ab5e49bdfda959c42aaf0aa3

It seems like it would be more reliable to explicitly launch chromedriver from an environment variable as well so we know exactly what is running.

cc @mdebbar