http2: fix flaky test-http2-https-fallback

The test was flaky because it relied on a specific order of
asynchronous operation that were fired paralellely. This was true
on most platform and conditions, but not all the time.

See: https://github.com/nodejs/node/pull/18986

PR-URL: https://github.com/nodejs/node/pull/19093


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>