Allow to change port for tests

[CyberTailor]

When port 8080 is busy, tests fail.
https://bugs.gentoo.org/901143

[victormihalache]

You can use the following run method to specify a custom port:

run(onRequest, initSettings(port=Port(2000)))

Note that it's normal behaviour for it to fail when the port is busy, any program will fail to use a port that's being used by another program.