[rmw] rmw_fastrtps_dds does not work well for C++ <-> Python pub/sub testing?

[EricCousineau-TRI]

In #285, I had to workaround testing expectations based on the effective RMW implementation being used: https://github.com/RobotLocomotion/drake-ros/blob/5e8b52e557f4f6b5c996f5239de8c106bfceb04c/drake_ros/drake_ros/test/core_test.py#L122-L169

This makes me concerned about using or even supporting rmw_fastrtps_cpp if something this simple is flaky.

Some options local to drake-ros: (a) Keep this pattern, just warn when rmw_cyclonedds_cpp is not in use whenever testing diverges. (b) Only ever allow rmw_cyclonedds_cpp, at least for our testing in drake-ros (and usage in Anzu).

\cc @adityapande-1995 @calderpg-tri @IanTheEngineer