Move message Boost theme behat tests to core_message behat tests

After some conversations, we decided to create some of the behat tests only for Boost theme and move them again to main core_message behat tests once Clean theme has been removed. The reason to do that is that right now behavior for Clean and Boost themes are not exactly the same.

This issue should we finished once Classic theme is added to core to be sure that group messaging behat tests are passing for Classic theme as well, and drawer behavior is the same for both Boost and Classic.