[tools/perf] Fix benchmark smoke tests to not set story_set.stories d…

…irectly

Without this fix, catapult roll is blocked (https://codereview.chromium.org/2547293002)
because CL https://codereview.chromium.org/2153513002/ changes story_set.stories
to be a property.

BUG=catapult-project/catapult#2429
TBR=eakuefner@chromium.org

Review-Url: https://codereview.chromium.org/2552543002
Cr-Commit-Position: refs/heads/master@{#436175}

tools/perf/benchmarks/benchmark_smoke_unittest.py

@@ -53,9 +53,10 @@ class SinglePageBenchmark(benchmark):  # pylint: disable=no-init
       def CreateStorySet(self, options):
         # pylint: disable=super-on-old-class
         story_set = super(SinglePageBenchmark, self).CreateStorySet(options)
-        for story in story_set.stories:
-          story_set.stories = [story]
-          break
+        # Only smoke test the first story since smoke testing everything takes
+        # too long.
+        for s in story_set.stories[1:]:
+          story_set.RemoveStory(s)
         return story_set
 
     # Set the benchmark's default arguments.

tools/perf/benchmarks/system_health_smoke_test.py

@@ -70,8 +70,11 @@ class SinglePageBenchmark(benchmark_class):  # pylint: disable=no-init
       def CreateStorySet(self, options):
         # pylint: disable=super-on-old-class
         story_set = super(SinglePageBenchmark, self).CreateStorySet(options)
-        assert story_to_smoke_test in story_set.stories
-        story_set.stories = [story_to_smoke_test]
+        stories_to_remove = [s for s in story_set.stories if s !=
+                             story_to_smoke_test]
+        for s in stories_to_remove:
+          story_set.RemoveStory(s)
+        assert story_set.stories
         return story_set
 
     options = GenerateBenchmarkOptions(benchmark_class)