expand the test for access to the abort signal within resolvers

GraphQL-Core already keeps the abort signal in GraphQLResolveInfo and already
had this test, so nothing carries over but a comment clarifying how it relates
to the upstream version, which subscribes to an "abort" event instead of
cancelling the resolver task.

Replicates graphql/graphql-js@566023c

Author: Cito

tests/execution/test_cancellation.py

@@ -145,12 +145,17 @@ async def provides_access_to_the_abort_signal_within_resolvers():
         )
 
         async def cancellable_async_fn(abort_signal):
-            # never reached: the resolver is cancelled before its body runs
+            # never reached: the resolver task is cancelled before its body runs
             raise await abort_signal.wait()  # pragma: no cover
 
         # Contrary to JavaScript, where the abort signal is passed as an additional
         # argument to the resolvers, in GraphQL-Core it is available via the resolve
-        # info, just like the context value.
+        # info, just like the context value. The GraphQL.js test that this mirrors
+        # subscribes to the signal's "abort" event and then throws an unrelated error
+        # to prove the abort error supersedes it; GraphQL-Core has no event listener
+        # and instead cancels the running resolver task, so the resolver awaits the
+        # signal directly and raises its reason (which the cancellation pre-empts) --
+        # this is race-safe because either path yields the same abort error.
         def resolve_id(info):
             return cancellable_async_fn(info.abort_signal)