Start test servers on a port the system picks (closes #211)

The internal/server and internal/ircserver tests picked a port by
opening a listener, reading its port and closing it. They then started
a server on that number. Another process could take the port in
between, and a test failed with "address already in use".

The IRC server tests now start the server on 127.0.0.1:0 and read the
port back from its listener. The HTTP server tests start the app with
port 0. The HTTP server now keeps the listener it binds, so the stop
test sends its long poll to the port the app was given.

Model: opus-5-5

internal/ircserver/server_test.go

@@ -137,20 +137,6 @@ func newTestEnvWithResolver(
 
 	brk := broker.New()
 
-	var listenConfig net.ListenConfig
-
-	listener, err := listenConfig.Listen(t.Context(), "tcp", "127.0.0.1:0")
-	if err != nil {
-		t.Fatalf("listen: %v", err)
-	}
-
-	addr := listener.Addr().String()
-
-	err = listener.Close()
-	if err != nil {
-		t.Fatalf("close listener: %v", err)
-	}
-
 	log := slog.New(slog.NewTextHandler(
 		os.Stderr,
 		&slog.HandlerOptions{Level: slog.LevelError}, //nolint:exhaustruct
@@ -160,7 +146,9 @@ func newTestEnvWithResolver(
 		log, cfg, database, brk, interval, deadline, resolve,
 	)
 
-	err = srv.Start(addr)
+	// Port 0 makes the system pick a free port. dial reads it
+	// back from the server's listener.
+	err = srv.Start("127.0.0.1:0")
 	if err != nil {
 		t.Fatalf("start irc server: %v", err)
 	}