Allow configuring test client's port

[nemanja-boric-sociomantic]

In case we have several tests running in parallel, each with the separate node, we need to make
client's connect to the different ports. This is possible by overriding DlsTestCase.prepare method, but it should be easier to configure this (probably via static variable, configurable from TestRunners).

See sociomantic-tsunami/dlsnode#7

[gavin-norman-sociomantic]

We want to release v13.2.0 soon. As this issue seems non-urgent, I've moved it to v13.3.0.