Fixes #37413 - Let the kernel assign a TCP port in tests

This binds on port 0, which lets the kernel decide which port to use.
The port is then read back from the listener. Because the kernel knows
which ports it's allowed to bind to, it removes the risk of a failure
due to collisions or policies.

test/test_helper.rb

@@ -39,19 +39,24 @@ def setup
   end
 
   def launch(protocol: 'https', plugins: [], settings: {})
-    port = 1024 + rand(63_000)
+    port = 0
     @settings = Proxy::Settings::Global.new(settings.merge("#{protocol}_port" => port))
     @t = Thread.new do
       launcher = Proxy::Launcher.new(@settings)
       app = launcher.public_send("#{protocol}_app", port, plugins)
-      launcher.webrick_server(app.merge(AccessLog: [Logger.new('/dev/null')]), ['localhost'], port).start
+      server = launcher.webrick_server(app.merge(AccessLog: [Logger.new('/dev/null')]), ['localhost'], port)
+      # Read back the actual port it bound to
+      @settings["#{protocol}_port"] = server.listeners[0].addr[1]
+      server.start
     end
     Timeout.timeout(2) do
-      sleep(0.1) until can_connect?('localhost', @settings.send("#{protocol}_port"))
+      sleep(0.1) until can_connect?('localhost', @settings["#{protocol}_port"])
     end
   end
 
   def can_connect?(host, port)
+    return false if port == 0
+
     TCPSocket.new(host, port).close
     true
   rescue Errno::ECONNREFUSED, Errno::EHOSTUNREACH