Fix benchmark test failures erroneously passing on CI (#9403)

Author: elliette

tool/ci/benchmark_performance.sh

@@ -10,5 +10,5 @@ set -ex
 source ./tool/ci/setup.sh
 
 pushd $DEVTOOLS_DIR/packages/devtools_app
-flutter test -j, --concurrency=1 benchmark/devtools_benchmarks_test.dart
+flutter test --concurrency=1 benchmark/devtools_benchmarks_test.dart
 popd