test: move http2 test to parallel

A fix to test-http2-session-timeout makes it sufficiently robust that it
can be moved to the parallel directory.

PR-URL: #24877
Fixes: #20628
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: Trott