Fix PSK invocation: GnuTLS PSK length (more)

Replace more sample PSK by longer (GnuTLS-compatible) strings, taking care
of keeping distinct PSK distinct for wrong-PSK tests.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 16e0a68..5fb954d 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -8470,7 +8470,7 @@
 
 run_test    "DTLS wrong PSK: badmac alert" \
             "$P_SRV dtls=1 psk=73776f726466697368 force_ciphersuite=TLS-PSK-WITH-AES-128-GCM-SHA256" \
-            "$P_CLI dtls=1 psk=abc124" \
+            "$P_CLI dtls=1 psk=73776f726466697374" \
             1 \
             -s "SSL - Verification of the message MAC failed" \
             -c "SSL - A fatal alert message was received from our peer"