Explicitly disable all DTLS options in tls13-only.h

This makes no difference when starting from the default configuration. It
allows tls13-only.h to be used with other base configurations such as `full`.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/configs/tls13-only.h b/tests/configs/tls13-only.h
index 1f212e7..38286d1 100644
--- a/tests/configs/tls13-only.h
+++ b/tests/configs/tls13-only.h
@@ -34,6 +34,7 @@
 #undef MBEDTLS_SSL_PROTO_DTLS
 #undef MBEDTLS_SSL_DTLS_ANTI_REPLAY
 #undef MBEDTLS_SSL_DTLS_HELLO_VERIFY
+#undef MBEDTLS_SSL_DTLS_SRTP
 #undef MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE
 #undef MBEDTLS_SSL_DTLS_CONNECTION_ID
 #undef MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT