refactor(MQTT Node): Refactor, fix duplicate triggers, and add Unit tests (#9847)

Co-authored-by: Elias Meire <elias@meire.dev>

packages/nodes-base/nodes/MQTT/test/GenericFunctions.test.ts

@@ -0,0 +1,38 @@
+import { MqttClient } from 'mqtt';
+import { mock } from 'jest-mock-extended';
+
+import { createClient, type MqttCredential } from '../GenericFunctions';
+
+describe('createClient', () => {
+	const mockConnect = jest.spyOn(MqttClient.prototype, 'connect').mockImplementation(function (
+		this: MqttClient,
+	) {
+		setImmediate(() => this.emit('connect', mock()));
+		return this;
+	});
+
+	beforeEach(() => jest.clearAllMocks());
+
+	it('should create a client with minimal credentials', async () => {
+		const credentials = mock<MqttCredential>({
+			protocol: 'mqtt',
+			host: 'localhost',
+			port: 1883,
+			clean: true,
+			clientId: 'testClient',
+			ssl: false,
+		});
+		const client = await createClient(credentials);
+
+		expect(mockConnect).toBeCalledTimes(1);
+		expect(client).toBeDefined();
+		expect(client).toBeInstanceOf(MqttClient);
+		expect(client.options).toMatchObject({
+			protocol: 'mqtt',
+			host: 'localhost',
+			port: 1883,
+			clean: true,
+			clientId: 'testClient',
+		});
+	});
+});