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 https://git.picodata.io/picodata/picodata/picodata/-/issues/65