test: improve test-https-server-keep-alive-timeout

The test is flaky under load. These changes greatly improve reliability.

* Use a recurring interval to determine when the test should end rather
  than a timer.
* Increase server timeout to 500ms to allow for events being delayed by
  system load

Changing to an interval has the added benefit of reducing the test run
time from over 2 seconds to under 1 second.

Fixes: https://github.com/nodejs/node/issues/13307

PR-URL: https://github.com/nodejs/node/pull/13312


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>