fix utf8 terminal benchmark

[jerch]

Fixes #5837.

Before:

   Context "out-test/benchmark/Terminal.benchmark.js"
      Context "Terminal: ls -lR /usr/lib"
         Context "write/string"
            Case "#1" : 5 runs - average throughput: 28.23 MB/s
         Context "write/utf8"
            Case "#1" : 5 runs - average throughput: 28.70 MB/s

After:

   Context "out-test/benchmark/Terminal.benchmark.js"
      Context "Terminal: ls -lR /usr/lib"
         Context "write/string"
            Case "#1" : 5 runs - average throughput: 27.98 MB/s
         Context "write/utf8"
            Case "#1" : 5 runs - average throughput: 33.17 MB/s