We would like to run flutter for web tests in MacOS. Since we will be moving our tests to LUCI, we we should also prepare the LUCI recipe.

Have the first LUCI recipe:

• running on macOS container (Mojave or Catalina)
• running felt tool
• with the 'licences' parameter under $ENGINE_PATH/src/flutter/lib/web_ui directory

example from cirrus CI: https://github.com/flutter/engine/blob/master/.cirrus.yml#L40