test: mark http2 tests as flaky on 10.x

These tests are already marked as flaky on 12.x and master.

PR-URL: https://github.com/nodejs/node/pull/31887


Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>