[graphql] defend against hang in backfill test (#26136)

Had an error cause these tests to hang indefinitely, prevent that from
happening again

## How I Tested These Changes

bk

python_modules/dagster-graphql/dagster_graphql_tests/graphql/test_partition_backfill.py

@@ -687,10 +687,13 @@ def test_cancel_asset_backfill(self, graphql_context):
 
             assert not result.errors
             assert result.data
+            assert result.data["launchPipelineExecution"]["__typename"] == "LaunchRunSuccess"
 
             # ensure the execution has happened
+            start = time.time()
             while not os.path.exists(path):
                 time.sleep(0.1)
+                assert time.time() - start < 60, "timed out waiting for file"
 
             result = execute_dagster_graphql(
                 graphql_context,
@@ -763,10 +766,13 @@ def test_cancel_then_retry_asset_backfill(self, graphql_context):
 
             assert not result.errors
             assert result.data
+            assert result.data["launchPipelineExecution"]["__typename"] == "LaunchRunSuccess"
 
             # ensure the execution has happened
+            start = time.time()
             while not os.path.exists(path):
                 time.sleep(0.1)
+                assert time.time() - start < 60, "timed out waiting for file"
 
             result = execute_dagster_graphql(
                 graphql_context,