fix: test running on macos

Pytest supports running tests in parallel using the xdist plugin. In order to support it Picodata should avoid port collision. It uses dedicated IP address 127.7.n.1 where n = xdist_worker_number. Unfortunately, it doesn't work on MacOS, because it doesn't provide any loopback aliases but original 127.0.0.1.

This patch provides another address generation logics. The subnet parameter is superseeded with a base_port, that is 3300 + n * 100. In this way, every pytest (xdist) worker gets a dedicated port range [3301, 3399], [3401, 3499] and so on.

Close #65 (closed)