test: fix flaky test-http2-settings-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.

Fixes: https://github.com/nodejs/node/issues/18251

PR-URL: https://github.com/nodejs/node/pull/19349


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>