Move mock_canvas to flutter_test package

[pingbird]

Many widget tests in the flutter package use packages/flutter/test/rendering/mock_canvas.dart as a convenient method of validating painting behaviors. This library is fully documented yet can't be imported externally.

I would like to use this library for some tests, without having to copy its implementation to my project.

Proposal

The mock_canvas library should be moved to flutter_test, where it can be used from anywhere.

[jonahwilliams]

I think we could possibly break out that library, but we'd need to spend time to take a pass over the API and documentation