Investigate "DTLS Proxy: 3d, (gnutls|openssl), fragmentation" intermittent failures

[ronald-cron-arm]

The ssl-opt.sh tests DTLS proxy: 3d, openssl client, fragmentation, DTLS proxy: 3d, openssl client, fragmentation, nbio, DTLS proxy: 3d, gnutls client, fragmentation and DTLS proxy: 3d, gnutls client, fragmentation, nbio=2 fail intermittently on the CI with a frequency that significantly impacts CI throughput. Thus we are going to disable them.

These test cases were introduced as part of the DTLS ClientHello reassembly work: #10576 (development) and #10623 (3.6).

This issue is to understand the intermittent failures and how we could re-enable the test cases.

We should also consider doing deterministic tests in test_suite_ssl of DTLS fragment re-ordering.

[gilles-peskine-arm]

This falls under the general topic of intermittent failures of DTLS 3d tests. However, there has been a sharp rise of these failures recently, maybe always on the fragmentation tests? There may be a distinct cause for the fragmentation tests that are failing more frequently, or a common cause that is more likely to happen with fragmentation tests.