Initial tests for SSDP and Zeroconf discovery providers

The test classes use the Xcode's `XCTest` framework, and can be run from the main project. Some of the private fields in the tested classes have been extracted into private headers to be able to mock them.
@iadnan, done. Please check commit 2e5621cfe0e3b09da27be4e3eb9376ddb81f055b.