Try /usr/bin/echo for testing

.. although it seems not an option to include this in prod.
RSsee #2765

Omitting redirecting stderr

Author: drwetter

testssl.sh

@@ -17656,7 +17656,7 @@ run_renego() {
                # s_client STDIN too early as the close could come at any time and race with the tear down of s_client.
                # See https://github.com/drwetter/testssl.sh/issues/2590
                # In this case the added iteration is harmless as it will just spin in backgroup
-               for ((i=0; i <= ssl_reneg_attempts; i++ )); do sleep $ssl_reneg_wait; echo R 2>/dev/null; k=0; \
+               for ((i=0; i <= ssl_reneg_attempts; i++ )); do sleep $ssl_reneg_wait; /usr/bin/echo R 2>/dev/null; k=0; \
                    # 0 means client is renegotiating & doesn't return an error --> vuln!
                    # 1 means client tried to renegotiating but the server side errored then. You still see RENEGOTIATING in the output
                    # Exemption from above: server closed the connection but return value was zero