test: fix flaky test-http-regr-gh-2928

Fix flaky test-http-regr-gh-2928 that has been failing on Raspberry Pi
devices in CI.

Fixes: https://github.com/nodejs/node/issues/4830
PR-URL: https://github.com/nodejs/node/pull/5154


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>