refactor: Split off ServerInitializer.

test/unit/init/ServerInitializer.test.ts

@@ -0,0 +1,18 @@
+import { ServerInitializer } from '../../../src/init/ServerInitializer';
+import type { HttpServerFactory } from '../../../src/server/HttpServerFactory';
+
+describe('ServerInitializer', (): void => {
+  const serverFactory: jest.Mocked<HttpServerFactory> = {
+    startServer: jest.fn(),
+  };
+
+  let initializer: ServerInitializer;
+  beforeAll(async(): Promise<void> => {
+    initializer = new ServerInitializer(serverFactory, 3000);
+  });
+
+  it('starts an HTTP server.', async(): Promise<void> => {
+    await initializer.handle();
+    expect(serverFactory.startServer).toHaveBeenCalledWith(3000);
+  });
+});