test: improve test-net-pingpong

This includes the following changes:
- a more strict data check rather than a regex
- reduced number of annoying log calls

The most important of the changes is the annoying log calls, which
speeds up the test execution from about 0m1.130s to 0m0.481s on my
machine.

PR-URL: https://github.com/nodejs/node/pull/2429


Reviewed-By: Rich Trott <rtrott@gmail.com>