add unit test for connect and disconnect

tests/on-apply.spec.ts

+import { App, Context } from 'koishi';
+import { InjectContext, OnApply, OnConnect, OnDisconnect, Provide } from '..';
+import { KoishiPlugin } from '../src/register';
+
+declare module 'koishi' {
+  // eslint-disable-next-line @typescript-eslint/no-namespace
+  namespace Context {
+    interface Services {
+      myPlugin: MyPlugin;
+    }
+  }
+}
+
+@Provide('myPlugin', { immediate: true })
+@KoishiPlugin()
+class MyPlugin implements OnApply, OnConnect, OnDisconnect {
+  @InjectContext()
+  ctx: Context;
+
+  applied = false;
+  connected = false;
+  disconnected = false;
+  onApply() {
+    this.applied = true;
+  }
+  onConnect() {
+    this.connected = true;
+  }
+  onDisconnect() {
+    this.disconnected = true;
+  }
+}
+
+describe('Apply and Connect', () => {
+  it('should be applied and connected', async () => {
+    const app = new App();
+    app.plugin(MyPlugin);
+    const myPlugin = app.myPlugin;
+    expect(myPlugin.applied).toBe(true);
+    expect(myPlugin.connected).toBe(false);
+    expect(myPlugin.disconnected).toBe(false);
+    await app.start();
+    expect(myPlugin.connected).toBe(true);
+    expect(myPlugin.disconnected).toBe(false);
+    await myPlugin.ctx.dispose();
+    expect(myPlugin.disconnected).toBe(true);
+    await app.stop();
+  });
+});