Get free port with test that the port is not in use.

[oisinmulvihill]

Hi Ed,

I've found the random port process currently being used was not working reliably on my home desktop. I didn't have a problem on my laptop strangely. I've solved this elsewhere in the evasion-common get_free_port() which I've adapted and would like to contribute here.

Original get_free_port()

• https://github.com/oisinmulvihill/evasion-common/blob/master/lib/evasion/common/net.py

All the best,

Oisin