Check that --graph works in CI

Author: eregon

.github/workflows/test.yml

@@ -33,7 +33,7 @@ jobs:
           for benchmark in $SKIP_BENCHMARKS; do
             rm -rf "benchmarks/${benchmark}.rb" "benchmarks/${benchmark}/"
           done
-          ./run_benchmarks.rb
+          ./run_benchmarks.rb --graph
         env:
           WARMUP_ITRS: '1'
           MIN_BENCH_ITRS: '1'