Avoid flaky test failures by retrying on local port conflicts (#15818)

This test obtains an available port number by calling `TCPServer.new`,
then closes it and passes the same port number as `local_port` to `TCPSocket.new`.

However, `TCPSocket.new` could occasionally fail with `Errno::EADDRINUSE`
at the bind(2) step.
I believe this happens when tests are run in parallel and another process
on the same host happens to bind the same port in the short window between
closing the `TCPServer` and calling `TCPSocket.new`.

To address this race condition, the test now retries with a newly selected
available port when such a conflict occurs.

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