psasim: add timeout while waiting for psa_server to start

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

tests/psa-client-server/psasim/test/start_server.sh

@@ -8,7 +8,14 @@ set -e
 # The server creates some local files when it starts up so we can wait for this
 # event as signal that the server is ready so that we can start client(s).
 function wait_for_server_startup() {
+    SECONDS=0
+    TIMEOUT=10
+
     while [ $(find . -name "psa_notify_*" | wc -l) -eq 0 ]; do
+        if [ "$SECONDS" -ge "$TIMEOUT" ]; then
+            echo "Timeout: psa_server not started within $TIMEOUT seconds."
+            return 1
+        fi
         sleep 0.1
     done
 }