test: improve performance of stringbytes test

String concatenation in the assert messages has drastic impact on test
runtime. Removal of these messages is unlikely to affect debugging if
any breaking changes are made.

Previous time to run:

    $ time ./iojs test/parallel/test-stringbytes-external.js

    real    0m2.321s
    user    0m2.256s
    sys     0m0.092s

With fix:

    $ time ./iojs test/parallel/test-stringbytes-external.js

    real    0m0.518s
    user    0m0.508s
    sys     0m0.008s

PR-URL: https://github.com/nodejs/node/pull/2544


Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: thefourtheye - Sakthipriyan Vairamani <thechargingvolcano@gmail.com>