test: tls13: change server output check

tls13 server now does not parse pre-shared key extension unless
there are some psk key exchange modes really available.
For `ephemeral_all/psk_or_ephemeral` configuration pairs, there
wouldn't be any psk key exchange mode available, so the check
of "Pre shared key found" should be inverse.

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>
diff --git a/tests/opt-testcases/tls13-kex-modes.sh b/tests/opt-testcases/tls13-kex-modes.sh
index 758da1d..ce10307 100755
--- a/tests/opt-testcases/tls13-kex-modes.sh
+++ b/tests/opt-testcases/tls13-kex-modes.sh
@@ -562,7 +562,7 @@
             -s "found pre_shared_key extension" \
             -S "Found PSK_EPHEMERAL KEX MODE" \
             -s "Found PSK KEX MODE" \
-            -s "Pre shared key found" \
+            -S "Pre shared key found" \
             -S "No matched PSK or ticket" \
             -S "key exchange mode: psk$"  \
             -S "key exchange mode: psk_ephemeral"  \
@@ -745,7 +745,7 @@
             -s "found pre_shared_key extension" \
             -s "Found PSK_EPHEMERAL KEX MODE" \
             -S "Found PSK KEX MODE" \
-            -s "Pre shared key found" \
+            -S "Pre shared key found" \
             -S "No matched PSK or ticket" \
             -S "key exchange mode: psk$"  \
             -S "key exchange mode: psk_ephemeral"  \
@@ -1425,7 +1425,7 @@
             -s "found pre_shared_key extension" \
             -s "Found PSK_EPHEMERAL KEX MODE" \
             -S "Found PSK KEX MODE" \
-            -s "Pre shared key found" \
+            -S "Pre shared key found" \
             -S "No matched PSK or ticket" \
             -S "key exchange mode: psk$"  \
             -S "key exchange mode: psk_ephemeral"  \