Write a knowledge base article about reproducible SSL testing

[gilles-peskine-arm]

The test programs ssl_client2 and ssl_server2 have an option reproducible=… which allows running them in a reproducible mode, where successive runs with the same options (and using the same or close-enough versions of the library) will result in the same traffic patterns. This is useful when you have a build of Mbed TLS that works and one that doesn't, for example with and without a cryptographic accelerator.