Health check test uses addresses nothing can listen on (closes #158)

TestHealthCheck closed the app's server, and later smallwebwaf, and
expected connections to the freed ports to fail. Another test could take
such a port in between, so the connection succeeded and the test failed.

The failing app check now uses the multicast address 224.0.0.1, to which
TCP cannot connect. The failing smallwebwaf check uses port 0, on which
nothing listens. Neither result depends on other processes. The code
under test is unchanged.

Model: opus-5-5

internal/smallwebwaf/healthcheck_test.go

@@ -60,8 +60,13 @@ func TestHealthCheck(t *testing.T) {
 		}, 0, "")
 	}
 
-	app.Close()
-	wantHealthCheck(t, env, 1, "unhealthy: connect to the app: ")
+	// Another test can take a closed server's port at once, so the app's
+	// address here is one that nothing can listen on: TCP cannot connect
+	// to a multicast address such as 224.0.0.1.
+	wantHealthCheck(t, map[string]string{
+		listenAddr:  ":" + port,
+		upstreamURL: "http://224.0.0.1",
+	}, 1, "unhealthy: connect to the app: ")
 
 	stop()
 
@@ -71,7 +76,10 @@ func TestHealthCheck(t *testing.T) {
 		t.Fatal("still running after being told to stop")
 	}
 
-	wantHealthCheck(t, env, 1, "unhealthy: ask smallwebwaf: ")
+	// Listening on port 0 takes any free port, so nothing listens on port 0
+	// itself.
+	wantHealthCheck(t, map[string]string{listenAddr: ":0"}, 1,
+		"unhealthy: ask smallwebwaf: ")
 	wantHealthCheck(t, map[string]string{listenAddr: "8080"}, 1,
 		"unhealthy: invalid setting: SWWAF_LISTEN_ADDR: ")
 }