Use systemctl to properly detect ephemeral ports on Mac OS (aka darwin) by fetching
systemctl values:
* net.inet.ip.portrange.first
* net.inet.ip.portrange.last
This will avoid the message:
`[INFO] freeport: ephemeral port range detection not configured for GOOS="darwin"`
and properly detect the correct port range
This should cut down on test flakiness.
Problems handled:
- If you had enough parallel test cases running, the former circular
approach to handling the port block could hand out the same port to
multiple cases before they each had a chance to bind them, leading to
one of the two tests to fail.
- The freeport library would allocate out of the ephemeral port range.
This has been corrected for Linux (which should cover CI).
- The library now waits until a formerly-in-use port is verified to be
free before putting it back into circulation.
