As we are migrating tests to luci for all repos, a single file to control tests is needed, similar to the yaml file for cirrus tests (flutter): https://github.com/flutter/flutter/blob/master/.cirrus.yml

On builder level, we will use try_builders.json in different repos by adding an enabled flag.

• add flag to engine
• add flag to framework
• add flag to cocoon
• add flag to packages
• updating cocoon backend to include enable/disable logic