Closes https://github.com/flutter/flutter/issues/161449.
~3 LOC, with 203 lines of tests (including an e2e integration test that
it actually works).
Feedback welcome!
(The reason I'm working on this is the ability to pass environment
variables makes it much easier and less hacky to make
`android_engine_test` configurable, i.e. have different expected outputs
for OpenGLES/Vulkan, compare screenshots locally for deflaking, etc).
