sock_stress: fix hvsock URL parsing

[djs55]

Previously a URL hvsock://00000000-0000-0000-0000-000000000000:00001010-FACB-11E6-BD58-64006A7986D3 would be rejected because url.Parse doesn't like the non-integer port.

Add a test case to demonstrate this, then add a fix to use the URL path, so hvsock://00000000-0000-0000-0000-000000000000/00001010-FACB-11E6-BD58-64006A7986D3

This is on top of #66 (add support for go modules)