"Connection refused" errors when attempting parallel test execution

[keturn]

During MovingBlocks/Terasology#4576 I noticed that running tests with --parallel sometimes resulted in an error Connection refused: localhost/127.0.0.1:25777, which I haven't seen in other test runs.

I didn't dig in to the details, but I'm guessing multiple processes were probably trying to use the same ports at once. If that's the case, either

a. they shouldn't be trying to use network ports at all, or
b. they need some smarter port-allocation logic that's muchmuch less likely to have collisions with other instances.

[jdrueckert]

resolve "connection refused" errors on parallel test execution