✅ add specified test for mirai adapter

tests/test_plugins/test_mirai.py

@@ -0,0 +1,13 @@
+from nonebot.plugin import on_message
+from nonebot.adapters.mirai import Bot, MessageEvent
+
+message_test = on_message()
+
+
+@message_test.handle()
+async def _message(bot: Bot, event: MessageEvent):
+    text = event.get_plaintext()
+    if not text:
+        return
+    reversed_text = ''.join(reversed(text))
+    await bot.send(event, reversed_text, at_sender=True)