Rare intermittent failure of DTLS 3d tests

This issue tracks rare intermittent failures of DTLS 3D (drop, delay, duplicate) tests in ssl-opt.sh on the CI.

In the merge queue for #8443 (ephemeral links: build, log), for full_without_ecdhe_ecdsa:

https://mbedtls.trustedfirmware.org/job/mbed-tls-pr-head/job/gh-readonly-queue%252Fdevelopment%252Fpr-8443-fc31cb28eeda7fafef1bd392fccee3b99035105b/1/

Logs: 8443-full_without_ecdhe_ecdsa.zip

Also observed on https://mbedtls.trustedfirmware.org/blue/organizations/jenkins/mbed-tls-pr-head/detail/PR-8454-head/7 in all.sh full_without_ecdhe_ecdsa.

Also observed on https://mbedtls.trustedfirmware.org/job/mbed-tls-pr-head/job/PR-8612-head/2/ PR-8612-head-2.zip