add scope unit test

tests/scope.spec.ts

+import { App, Session } from 'koishi';
+import { OnGuild, OnPlatform, Registrar, UseCommand } from 'koishi-decorators';
+import { DefinePlugin } from '../src/register';
+
+@OnPlatform('discord')
+@DefinePlugin()
+class MyClass {
+  @OnGuild('1111111111')
+  @UseCommand('foo')
+  foo() {}
+}
+
+describe('Scope', () => {
+  let app: App;
+
+  beforeEach(async () => {
+    app = new App();
+    await app.start();
+  });
+
+  it('should check scope', () => {
+    const correctSession = {
+      guildId: '1111111111',
+      platform: 'discord',
+    } as Session;
+
+    const wrongSession1 = {
+      guildId: '2222222222',
+      platform: 'discord',
+    } as Session;
+
+    const wrongSession2 = {
+      guildId: '1111111111',
+      platform: 'telegram',
+    } as Session;
+
+    app.plugin(MyClass);
+    const methodCtx = app.command('foo').context;
+
+    expect(methodCtx.filter(correctSession)).toBe(true);
+    expect(methodCtx.filter(wrongSession1)).toBe(false);
+    expect(methodCtx.filter(wrongSession2)).toBe(false);
+  });
+});