Replace howsmyssl.com with letsencrypt.org in the https_x509_bundle
example. howsmyssl.com is a third-party server that is frequently
unreachable from CI, causing flaky test failures. letsencrypt.org
chains to the same ISRG Root X1 CA, so the custom certificate bundle
validation coverage is identical.
Since letsencrypt.org was already present in the full bundle URL list,
remove the duplicate entry and reduce MAX_URLS from 9 to 8. All 6
unique root CAs in the stress test are still covered.
For the QEMU stress test, increase per-connection timeout from 30s to
60s and final completion timeout from 60s to 180s. QEMU emulated
network is 3-5x slower than real hardware for TLS handshakes.
Add flaky markers to hardware tests to handle intermittent CI lab DHCP
failures that affect all Ethernet-based tests.