add testing source

dev/extras.ts

-import { Context } from 'koishi';
+import { Awaitable, Context } from 'koishi';
+import { PicResult, PicSource } from '../src';
+
+class TestPicsource extends PicSource {
+  constructor(ctx: Context, public name: string) {
+    super(ctx);
+  }
+  isDefault = true;
+  randomPic(picTags: string[]): Awaitable<PicResult> {
+    return {
+      url: `https://cdn02.moecube.com:444/images/ygopro-images-${this.name}/${
+        picTags[0] || '10000'
+      }.jpg`,
+      description: picTags[0] || '10000',
+    };
+  }
+}
 
 export default class ExtrasInDev {
-  constructor(ctx: Context) {}
+  constructor(ctx: Context) {
+    ctx.pics.addSource(new TestPicsource(ctx, 'zh-CN'));
+    ctx.pics.addSource(new TestPicsource(ctx, 'en-US'));
+  }
+
+  static using = ['pics'] as const;
 }