fix tornado dev server

Author: rmorshea

src/idom/backend/tornado.py

@@ -10,6 +10,7 @@
 
 from tornado.httpserver import HTTPServer
 from tornado.httputil import HTTPServerRequest
+from tornado.ioloop import IOLoop
 from tornado.log import enable_pretty_logging
 from tornado.platform.asyncio import AsyncIOMainLoop
 from tornado.web import Application, RequestHandler, StaticFileHandler
@@ -67,6 +68,9 @@ async def serve_development_app(
 ) -> None:
     enable_pretty_logging()
 
+    # There might be a prior loop installed in a prior test case
+    IOLoop.clear_current()
+
     # setup up tornado to use asyncio
     AsyncIOMainLoop().install()