A gRPC test no longer loses its port to another process

The trusted-endpoint test needed two free network ports before it could start its server, and it found them the usual way: open a socket, ask the operating system which port it assigned, then close it again. Between that close and the server's own bind, the port belonged to nobody — so anything else on the machine could take it, and the server then refused to start with address already in use. On a quiet machine the gap is invisible; on a loaded build agent it is wide enough to catch, which is how the failure kept appearing on pull requests that had not touched the code at all.

The test now keeps the socket it opened and hands that same socket to the server, so the port is owned continuously and there is no moment for anything else to claim it. A new check runs the losing interleaving on purpose — a rival process grabbing the port at exactly the wrong instant — and fails if the port is ever left unowned again.