test: refactor test-http-exit-delay

test-http-exit-delay was introduced to confirm the removal of a 1 second
delay that can occur when exiting node after an http request.

This change refactors the test for simplicity and also in the hopes of
either eliminating flakiness on CI or, if not that, at least making
the source of the flakiness easier to track down.

Ref: https://github.com/nodejs/node/commit/16b59cbc
Fixes: https://github.com/nodejs/node/issues/4045
PR-URL: https://github.com/nodejs/node/pull/4055


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