test(ipc): simplify mocking

Author: KatoakDR

electron/main/game/__mocks__/game-instance.mock.ts

@@ -0,0 +1,8 @@
+import type { Mocked } from 'vitest';
+import { vi } from 'vitest';
+import type { Game } from '../game.instance.js';
+
+export class GameInstanceMock implements Mocked<typeof Game> {
+  getInstance = vi.fn();
+  newInstance = vi.fn();
+}