test: added string_decoder.js a parallel test

PR-URL: https://github.com/nodejs/node/pull/15923
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-benchmark-string_decoder.js

@@ -0,0 +1,7 @@
+'use strict';
+
+require('../common');
+
+const runBenchmark = require('../common/benchmark');
+
+runBenchmark('string_decoder', ['n=1']);