[Breaking Change] Integration tests golden file comparisons

[Matan Lurey]

Unless a user-defined goldenFileComparator is set, either manually in a test, or using a flutter_test_config.dart file, Android and iOS devices and their respective emulators/simulators have a new default that proxies to the local host filesystem, fixing a long-standing bug (#143299).

When will the change happen?

On master branch as of 4ff2deafd4b9d9b8bf030db8b2163b49ecdccf10 this change is already live, and we expect it to be available in the next stable release (not a hot-fix). Note again this change is for Android and iOS devices and emulators only, there are no changes for other platforms (for example, on the web, a similar technique is already used).

How to prepare for this change?

We expect the vast majority of users to have to do nothing, this feature is in practice fixing a long-standing bug. Some users may have written custom implementations; see the migration guide if you'd like to instead use the newly fixed implementation.