Clean up psk_list in ssl-opt.sh

[gilles-peskine-arm]

In ssl-opt.sh (3.6, 4.x), we set up psk_list with unintended values: the argument to psk_list= should be a comma-separated list of byte strings in hexadecimal, so Client_identity is wrong (I guess non-hex-digits are skipped?) and abc and def are weird (I guess an extra odd digit is either completed with a 0 or ignored?). Fix this.

Note that we need to be consistent with other psk_list and psk settings in individual test cases, since it matters whether the client's psk and the server's psk are in the server's psk_list. (On the server, psk and psk_list go through two different APIs: mbedtls_ssl_conf_psk and mbedtls_ssl_conf_psk_cb.)

PSK strings used for testing should be at least 4 bytes long (i.e. at least 8 hex digits) because GnuTLS rejects shorter strings.