Right now, customer_testing pulls ~tip-of-tree of the flutter/tests repo

This causes Flutter CI tree redness due to out-of-band updates to the flutter/tests repo.

We can solve the out-of-band breakage by pinning flutter/tests in flutter/flutter.

We could do that by adding a file to flutter/flutter (next to that ci.sh script) that contains the flutter/tests commit hash to pull. Then in ci.sh, we'd pull that commit hash instead of computing the commit hash (which is usually tip-of-tree flutter/tests) with the current script.

How to avoid flutter/tests hash becoming stale? We can add text to the flutter/tests PR template loudly asking committers there to activate their flutter/tests change by updating the hash checked into the file in flutter/flutter.

cc @johnmccutchan @jtmcdole @Piinks @matanlurey