test: restore full coverage for aborted resolver tests

The with_abort_signal helper added for the #4267 port cancels a
pre-aborted async resolver before its body runs; because GraphQL-Core
coroutines are lazy (unlike the eager resolvers in graphql-js, whose
bodies execute before the wrapped promise rejects), the resolver bodies
in the abort-before-await cancellation tests became unreachable and
dropped coverage below 100%. Mark them no-cover, mirroring the c8-ignore
graphql-js puts on the analogous never-called callbacks.

Author: Cito

tests/execution/test_abort_signal.py

@@ -100,7 +100,9 @@ async def stops_the_execution_when_aborted_during_object_field_completion():
         )
 
         async def todo(_info):
-            return {
+            # never reached: a triggered abort signal cancels this resolver
+            # before its body runs (unlike the eager resolvers in graphql-js)
+            return {  # pragma: no cover
                 "id": "1",
                 "author": must_not_be_called,
             }
@@ -143,7 +145,8 @@ async def provides_access_to_the_abort_signal_within_resolvers():
         )
 
         async def cancellable_async_fn(abort_signal):
-            raise await abort_signal.wait()
+            # never reached: the resolver 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
@@ -192,7 +195,9 @@ async def stops_the_execution_when_aborted_during_completion_with_custom_error()
         )
 
         async def todo(_info):
-            return {
+            # never reached: a triggered abort signal cancels this resolver
+            # before its body runs (unlike the eager resolvers in graphql-js)
+            return {  # pragma: no cover
                 "id": "1",
                 "author": must_not_be_called,
             }
@@ -239,7 +244,9 @@ async def stops_the_execution_when_aborted_during_completion_with_custom_string(
         )
 
         async def todo(_info):
-            return {
+            # never reached: a triggered abort signal cancels this resolver
+            # before its body runs (unlike the eager resolvers in graphql-js)
+            return {  # pragma: no cover
                 "id": "1",
                 "author": must_not_be_called,
             }
@@ -283,7 +290,8 @@ async def stops_the_execution_when_aborted_during_nested_object_completion():
         )
 
         async def author(_info):
-            return must_not_be_called
+            # never reached: the resolver is cancelled before its body runs
+            return must_not_be_called  # pragma: no cover
 
         awaitable_result = execute(
             schema,
@@ -423,7 +431,9 @@ async def stops_the_execution_when_aborted_with_proper_null_bubbling():
         )
 
         async def non_nullable_todo(_info):
-            return {
+            # never reached: a triggered abort signal cancels this resolver
+            # before its body runs (unlike the eager resolvers in graphql-js)
+            return {  # pragma: no cover
                 "id": "1",
                 "author": must_not_be_called,
             }