test: make stdout buffer test more robust

test-stdout-buffer-flush-on-exit is unfortunately non-deterministic. It
will, every so often, pass when it is supposed to fail. This is
currently guarded against by running the test with three different long
strings. This change increases it to five to reduce the false negatives.

PR-URL: https://github.com/nodejs/node/pull/6633


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>