test: reduce benchmark test run time

The `millions` argument was missing.

PR-URL: https://github.com/nodejs/node/pull/18787
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/sequential/test-benchmark-buffer.js

@@ -13,6 +13,7 @@ runBenchmark('buffers',
                'encoding=utf8',
                'endian=BE',
                'len=2',
+               'millions=0.000001',
                'method=',
                'n=1',
                'noAssert=true',