Bikeshed: separating rosapi_msgs and rosbridge_test_msgs

[Jacob Bandes-Storch]

Due to a change in rosidl_python, in order to get rosbridge_suite release for Rolling we will need to move the .msg/.srv definitions from `rosapi` and `rosbridge_library` into separate packages. I have a PR open to do this here: https://github.com/RobotWebTools/rosbridge_suite/pull/665

The messages from rosbridge_library were only used in testing, so I'm thinking of making a package called `rosbridge_test_msgs`. The only snag is that we will have to actually publish this package in a release so that it can be used for running rosbridge_library's tests on the build farm. It would be nice to keep the package used in testing only, but it sounds like that's not possible.

Does anyone have concerns with releasing a new package called `rosbridge_test_msgs` as part of rosbridge_suite? Or any suggestions for a better approach that avoids exposing internals of the library's tests?

Jacob