Skipping multi-RMW tests defined in test_communication for rmw_zenoh

[Yadunund]

As we look to run CI with rmw_zenoh, a non-DDS based middleware which would not support any cross-vendor tests, we'll need a way to skip running multi-RMW tests part of test_communication. At present system_tests/test_communication sets SKIP_MULTI_RMW_TESTS to OFF by default while explictly excluding certain cross-vendor tests for certain DDS middleware combinations.

If we add rmw_zenoh to the ros2.repos file and run CI, we can expect failures across all cros-vendor tests.

One "easy" way to resolve this is to add further exceptions to the CMakeLists.txt linked above for rmw_zenoh.

Another suggestion from @cottsay is to be able to infer "groups" of RMWs that are expected to be compatible on a per-test basis.

Related to #429.