Do async_setup_platform in background (#36244)

Co-authored-by: J. Nick Koston <nick@koston.org>

tests/components/demo/test_remote.py

@@ -22,6 +22,7 @@ class TestDemoRemote(unittest.TestCase):
         assert setup_component(
             self.hass, remote.DOMAIN, {"remote": {"platform": "demo"}}
         )
+        self.hass.block_till_done()
 
     # pylint: disable=invalid-name
     def tearDown(self):