GossipRouter tests: port conflicts as every instance uses 12001 by default

We have 4 tests involving GossipRouter. They use either tunnel.xml or tcpgossip.xml, which have GR running at 12001. When 2 or more of those tests run in parallel, we get a lot of SKIPPED tests, due to the fact the a GR cannot be started as its port is already taken (in setup()).
SOLUTION: instead of using tunnel.xml and tcpgossip.xml, create a stack programmatically, using a port for GR that's retrieved via the ResourceManager.
This also has the advantage that the GR tests don't rely on config files, which might get changed.