Consider ResourceWarnings as errors for shutdown test

Trinity shutdowns are improving which means we ocassionally
get shutdowns that are considered clean which then causes
our xfail to fail. Let's consider ResourceWarnings as
errors, too.

tests/integration/helpers.py

@@ -15,6 +15,7 @@ async def scan_for_errors(async_iterable):
 
     error_trigger = (
         "exception was never retrieved",
+        "ResourceWarning: unclosed resource",
         "Task was destroyed but it is pending",
         "Traceback (most recent call last)",
     )