test: simplify parallel/test-stringbytes-external
test: simplify parallel/test-stringbytes-external

Make the algorithm that creates the big input strings a little easier
to comprehend.  No functional changes, the string lengths are unchanged.

PR-URL: https://github.com/iojs/io.js/pull/1042
Reviewed-By: Trevor Norris <9cb5491eb5770dd8e0bf77a8a0335abf5f0488c5@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node