[PR #8457/db1b9129 backport][3.9] Use async fixture in testing docs (#8460)

**This is a backport of PR #8457 as merged into master
(db1b912).**

Co-authored-by: Sam Bull <[email protected]>

Author: patchback[bot]

docs/testing.rst

@@ -111,11 +111,11 @@ app test client::
             body='value: {}'.format(request.app[value]).encode('utf-8'))
 
     @pytest.fixture
-    def cli(loop, aiohttp_client):
+    async def cli(aiohttp_client):
         app = web.Application()
         app.router.add_get('/', previous)
         app.router.add_post('/', previous)
-        return loop.run_until_complete(aiohttp_client(app))
+        return await aiohttp_client(app)
 
     async def test_set_value(cli):
         resp = await cli.post('/', data={'value': 'foo'})