test: remove race condition in http flood test

Timer race results in some flakiness on slower devices in CI. Remove
unneeded setTimeout() and replace booleans with common.mustCall().

PR-URL: https://github.com/nodejs/node/pull/4793


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