test: remove excessively poor performance
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
R=@bnoordhuis