test: fix flaky test-http2-ping-flood

The test is unreliable on some Windows platforms in its current form.
Make it more robust by using `setInterval()` to repeat the flooding
until an error is triggered.

PR-URL: https://github.com/nodejs/node/pull/19395


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>