Fix PSK invocation: GnuTLS prompting

When given a PSK key but no username, gnutls-cli prompts for a password.
Prevent that by passing --pskusername with the same identity that
ssl_server2 uses by default.

Signed-off-by: Gilles Peskine <[email protected]>

tests/ssl-opt.sh

@@ -502,7 +502,7 @@ adapt_cmd_for_psk () {
     case "$2" in
         *openssl*s_server*) s='-psk abc123 -nocert';;
         *openssl*) s='-psk abc123';;
-        *gnutls-*) s='--pskkey=abc123';;
+        *gnutls-*) s='--pskusername=Client_identity --pskkey=abc123';;
         *) s='psk=abc123';;
     esac
     eval $1='"$2 $s"'