fix logic

cylc/flow/task_events_mgr.py

@@ -1626,17 +1626,17 @@ def process_execution_polling_intervals(
 
         # Basic example:
         >>> this([40, 40], 100, [10])
-        [40, 40, 20, 10]
+        [60, 40, 10]
 
         # Second 40 second delay gets lopped off the list because it's after
         # the execution time limit:
         >>> this([40, 40], 60, [10])
-        [40, 20, 10]
+        [60, 10]
 
         # Expand last item in exection polling intervals to fill the
         # execution time limit:
         >>> this([5, 20], 100, [10])
-        [5, 20, 20, 20, 20, 15, 10]
+        [20, 20, 20, 20, 20, 10]
 
         # There are no execution polling intervals set - polling starts
         # at execution time limit:
@@ -1660,7 +1660,10 @@ def process_execution_polling_intervals(
         # delay to get to the execution_time_limit
         remaining_time_to_time_limit = time_limit - sum(delays)
         if remaining_time_to_time_limit:
-            delays.append(remaining_time_to_time_limit)
+            if delays:
+                delays[0] += remaining_time_to_time_limit
+            else:
+                delays = [remaining_time_to_time_limit]
 
         # After the execution time limit poll at execution time limit polling
         # intervals.