test: reduce run time for string_decoder benchmark

test-benchmark-string_decoder was timing out in CI. Reduce the run time
by sending appropriate options such that each benchmark file only runs
one combination of options.

PR-URL: https://github.com/nodejs/node/pull/16118
Reviewed-By: Refael Ackermann <refack@gmail.com>

test/parallel/test-benchmark-string_decoder.js

@@ -4,4 +4,7 @@ require('../common');
 
 const runBenchmark = require('../common/benchmark');
 
-runBenchmark('string_decoder', ['n=1']);
+runBenchmark('string_decoder', ['chunk=16',
+                                'encoding=utf8',
+                                'inlen=32',
+                                'n=1']);