ssl_helpers: Restore rng_seed incrementation

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/src/test_helpers/ssl_helpers.c b/tests/src/test_helpers/ssl_helpers.c
index 860b0a3..53003b2 100644
--- a/tests/src/test_helpers/ssl_helpers.c
+++ b/tests/src/test_helpers/ssl_helpers.c
@@ -43,7 +43,7 @@
     mbedtls_test_handshake_test_options *opts)
 {
 #if defined(MBEDTLS_SSL_HANDSHAKE_WITH_CERT_ENABLED)
-    int rng_seed = 0xBEEF;
+    static int rng_seed = 0xBEEF;
 
     srand(rng_seed);
     rng_seed += 0xD0;