Change the way testharness unit tests start the server #29344
This is generic logic to check that all the server processes have started, which seems useful to any consumer trying to start multiple servers in the background. Therefore it makes more sense for it to live in the serve module than in wptrunner.
We previously imported the serve module to build the config, but then actually ran the server using `wpt serve` as a subprocess. This meant we had to re-implement the logic to check the servers were started and got it wrong (in particular, we were checking the http server but using the https server for tests). I removed the start() method and just had the constructor do the connection because using the class in an unconnected state doesn't make sense.
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