PING a quiet IRC listener client before closing it (closes #155)

serve closed a connection after five minutes with nothing read and never
sent a PING, so a quiet registered client lost its session. It now sends
PING :<server> after pingInterval with nothing read, and closes the
connection when nothing is read within pongDeadline after that. Every
line read starts both waits again. readTimeout is removed.

Unregistered connections are sent the PING too, so a PONG before
registration is now ignored instead of answered with ERR_NOTREGISTERED.

Model: opus-5-5

internal/ircserver/server_test.go

@@ -438,20 +438,35 @@ func TestPingPong(t *testing.T) {
 // TestQuietClientAnsweringPingStaysConnected checks that a
 // registered client that sends nothing is sent a PING each
 // time the ping interval passes, and stays connected as long
-// as it answers each one.
+// as it answers each one, whether with a PONG or with another
+// line.
 func TestQuietClientAnsweringPingStaysConnected(t *testing.T) {
 	t.Parallel()
 
-	env := newTestEnvWithShortPings(t)
-	client := env.dial(t)
-	client.register("answers")
-
-	for range 3 {
-		client.readUntil(isServerPing)
-		client.send("PONG :" + testServerName)
+	tests := []struct {
+		name   string
+		answer string
+	}{
+		{name: "PONG", answer: "PONG :" + testServerName},
+		{name: "PING of its own", answer: "PING :answer"},
 	}
 
-	client.sendAndExpect("PING :still here", "PONG")
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			env := newTestEnvWithShortPings(t)
+			client := env.dial(t)
+			client.register("answers")
+
+			for range 3 {
+				client.readUntil(isServerPing)
+				client.send(tt.answer)
+			}
+
+			client.sendAndExpect("PING :still here", "still here")
+		})
+	}
 }
 
 // TestQuietClientNotAnsweringPingIsClosed checks that a