Add platform specific unit tests

[LasseRosenow]

Now that we start to have very platform specific implementations for NetworkChannels such as the CoapUdpIpChannel for RIOT OS, it makes sense that we add unit tests that are executed on the correct runtime.

I am not yet sure how to integrate the CMAKE stuff to run make based RIOT OS tests, but I think we should find a way.

[erlingrj]

Yes, agreed. The first challenge is finding a nice test environment, such as RIOT with the tap interface

[LasseRosenow]

For Zephyr OS this is mostly the same as for RIOT, which is really nice: https://docs.zephyrproject.org/latest/connectivity/networking/native_sim_setup.html