Temporarily disable Cocoon test to unblock flutter/flutter#138521 (#346)

The code change in flutter/flutter#138521 causes [some golden diffs](https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8759248233367002865/+/u/run_test.dart_for_customer_testing_shard_and_subshard_None/test_stdout#:~:text=%7C%2000%3A04%20%2B36%3A%20/b/s/w/ir/x/t/flutter_customer_testing.flutter_cocoon.XTTDYM/tests/dashboard/test/widgets/commit_box_test.dart%3A%20CommitBox%20shows%20information%20correctly). The diffs are small and will be fixed once #138521 is landed.