test: relax timing in test-http-exit-delay

This test was originally intended to guard against regressions for
commit https://github.com/joyent/node/commit/16b59cbc74c8fe2f8b30f3af4c2f885b7bfb6030.

As such, it only needs to ensure that process exit has not been held up
by the date cache timer, which would fire on the next second.