Register our tests in Flutter's "customer tests" suite

[gnprice]

Flutter upstream has a scheme for Flutter users' test suites to be integrated as part of Flutter's own CI, in the "customer tests" suite:
https://github.com/flutter/tests

This has the effect that a Flutter change which breaks those tests is treated as a "breaking change", the same category as a change which breaks Google's own internal tests:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
That in turn sets a higher bar for being worth the breakage and calls for more effort from the upstream author of the change to help developers migrate across it.

We should register for this scheme, now that we're out of the experimental phase and are committed to Flutter. The main prerequisite is that we'll need to have some appropriate CI scripts in the first place:

• Set up CI: test, analyze, and generate #60

(The two major barriers that I suspect are the reasons most Flutter developers don't participate are (a) if your code isn't open-source, then it gets logistically complicated to do so, and (b) it necessarily requires you to be keeping up with Flutter main/master, rather than only upgrading on stable releases. But we're open-source and are already keeping up with Flutter main, which we value because it lets us make upstream changes and promptly benefit from them.)

I believe there are two occasions so far where this would have flagged an upstream change as breaking:

• Make SlottedMultiChildRenderObjectWidgetMixin a concrete class flutter/flutter#126108 (see our commit 2f0f469 )
• Add applyFocusChangesIfNeeded, have menus restore focus before activating flutter/flutter#130536 (see chat thread about breakage)