WIP: Zeroconf unit testing

For discussion. Unit tests in test-zeroconf.cpp defined with Catch. This PR will not compile in master but will if merged into e.g. 6d415e16.

• unit testing of the Zeroconf container
• advertise and browse local targets ⚠️
• browse remote targets
• stop advertising a local target

⚠ Since the basic Aseba::Zeroconf object is single-threaded, it can't advertise and browse at the same time!