Update broker_test.go to create a different broker with a new port for each test

[mikegfink]

Currently a single broker is used for the test suite, this may cause state based test failures. If a broker could be initialized to listen on different ports for each test then the tests will be better encapsulated.

Possible issue with knowing which port to connect to as data is stored in a suite variable. Tests do not appear to be run sequentially. This seems to be a function of the go testing package rather than go-check.