Unit tests cannot be run while kind-voltha is running

Running `make test` while a kind voltha environment is also running leads to a port conflict.

Suspect it may be due to the server.go tests, which use port 55555: https://gerrit.opencord.org/#/c/15648/2/internal/pkg/afrouter/server_test.go

David found a link that describes a technique for picking an unused available port: https://stackoverflow.com/questions/43424787/how-to-use-next-available-port-in-http-listenandserve