Check that the port is actually set

Not sure if it really can happen, but if there is no NetworkConnector
in the list of connectors, the port will never be set.
Make the listening port available in JUnit rule
There is no need to sleep between retries
Increase the max tries to 10

After removing the sleep one can just as well try some more.
Fix wording in exception ('retries' --> 'tries')
Merge pull request #159 from finn-no/make_port_available_in_junit_rule

Make the listening port available in JUnit rule
Merge pull request #161 from finn-no/improve_retries

Improve retries
Merge pull request #160 from finn-no/check_local_port_is_set

Check that the port is actually set