docs(rabbitmq): add testing section to rabbitmq README (#736)

Add testing documentation on how to prevent connection on test environment and how to mock rabbitmq
module derivated

packages/rabbitmq/README.md

@@ -38,6 +38,9 @@
     - [Competing Consumers](#competing-consumers)
     - [Handling errors](#handling-errors)
     - [Handling errors during queue creation](#handling-errors-during-queue-creation)
+  - [Testing](#testing)
+    - [Prevent Connection Error](#prevent-connection-error)
+    - [Mock Connection](#mock-connection)
   - [Contribute](#contribute)
   - [License](#license)
 
@@ -601,6 +604,83 @@ You have the option to use `forceDeleteAssertQueueErrorHandler` which will try t
 
 Obviously, you can also provide your own function and do whatever is best for you, in this case the function must return the name of the created queue.
 
+## Testing
+
+### Prevent Connection Error
+
+Normally, you would not want to setup a RabbitMq broker locally or even connect to a external one during tests. So, first we have to prevent the connection to be attempted. This can be easily achieved by passing an `undefined` config to `RabbitMQModule.forRoot`:
+
+```typescript
+import { RabbitMQModule } from '@golevelup/nestjs-rabbitmq';
+
+@Module({
+  imports: [
+    RabbitMQModule.forRoot(
+      RabbitMQModule,
+      /** Not sending config object makes the connection process to be ignored */
+      process.env.NODE_ENV !== 'test'
+        ? {
+            exchanges: [
+              {
+                name: 'exchange1',
+                type: 'topic',
+              },
+            ],
+            uri: 'amqp://rabbitmq:rabbitmq@localhost:5672',
+            connectionInitOptions: { wait: false },
+          }
+        : undefined
+    ),
+  ],
+})
+export class RabbitExampleModule {}
+```
+
+In this example, when running on a `test` environment, config will be `undefined` thus connection will no happen.
+
+### Mock Connection
+
+You saw above how to prevent the connection during test, but now we'll have also to mock the `RabbitExampleModule` so it exports `AmqpConnection` to our publishers (we do not worry about the subscribers because they will be ignored along with the connection).
+
+```typescript
+// In your test setup...
+
+import { Module } from '@nestjs/common';
+import { AmqpConnection } from '@golevelup/nestjs-rabbitmq';
+import { createMock } from '@golevelup/ts-jest';
+import { Test } from '@nestjs/testing';
+import { AppModule } from 'where your root module is located';
+import { RabbitExampleModule } from 'where your rabbitmq module is located';
+
+// Create a valid mock module
+@Module({
+  providers: [
+    {
+      provide: AmqpConnection,
+      useValue: createMock<AmqpConnection>(),
+    },
+  ],
+  exports: [AmqpConnection],
+})
+class MockRabbitExampleModule {}
+
+// Then override the real `RabbitMqModule` with the mocked one
+beforeAll(async () => {
+  const moduleFixture = await Test.createTestingModule({
+    imports: [AppModule],
+  })
+    .overrideModule(RabbitExampleModule)
+    .useModule(MockRabbitExampleModule)
+    .compile();
+
+  const app = moduleFixture.createNestApplication();
+
+  await app.init();
+});
+```
+
+And now your publishers that inject `AmqpConnection` will find it very well mocked.
+
 ## Contribute
 
 Contributions welcome! Read the [contribution guidelines](../../CONTRIBUTING.md) first.