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

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=https://github.com/catapult-project/catapult/issues/2429
TBR=eakuefner@chromium.org

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