test: reduce benchmark cases in test-benchmark-buffer

Specify `buffer=fast` so that we only run that and not that along with
`buffer=slow` in two benchmarks.

PR-URL: https://github.com/nodejs/node/pull/17111
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/sequential/test-benchmark-buffer.js

@@ -8,6 +8,7 @@ runBenchmark('buffers',
              [
                'aligned=true',
                'args=1',
+               'buffer=fast',
                'encoding=utf8',
                'len=2',
                'method=',