test: fix flaky test-https-client-get-url

Fixed test-https-client-get-url by waiting on HTTPS GET requests
to finish before closing the server.

PR-URL: https://github.com/nodejs/node/pull/12876
Fixes: https://github.com/nodejs/node/issues/12873


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>