Rename available values for tls13_kex_modes

Rename `psk_pure` to `psk` and `ephemeral_pure` to `ephemeral`

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 56c4a5f..6066bc7 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -1458,8 +1458,8 @@
 # ssl_client2/ssl_server2 example programs works.
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
 run_test    "TLS 1.3, key exchange mode parameter passing: PSK only" \
-            "$P_SRV tls13_kex_modes=psk_pure" \
-            "$P_CLI tls13_kex_modes=psk_pure" \
+            "$P_SRV tls13_kex_modes=psk" \
+            "$P_CLI tls13_kex_modes=psk" \
             0
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
 run_test    "TLS 1.3, key exchange mode parameter passing: PSK-ephemeral only" \
@@ -1468,8 +1468,8 @@
             0
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
 run_test    "TLS 1.3, key exchange mode parameter passing: Pure-ephemeral only" \
-            "$P_SRV tls13_kex_modes=ephemeral_pure" \
-            "$P_CLI tls13_kex_modes=ephemeral_pure" \
+            "$P_SRV tls13_kex_modes=ephemeral" \
+            "$P_CLI tls13_kex_modes=ephemeral" \
             0
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
 run_test    "TLS 1.3, key exchange mode parameter passing: All ephemeral" \