test: minimize test-http-get-pipeline-problem

Reduce resoures required by test.

Clarify comment explaining source of test and what the test is looking
for.

Fixes: https://github.com/nodejs/node/issues/5725
PR-URL: https://github.com/nodejs/node/pull/5728


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>