Avoid static buffer in debug module

Caused issues in threading situations
diff --git a/tests/suites/test_suite_debug.function b/tests/suites/test_suite_debug.function
index 5c8993f..6adcc10 100644
--- a/tests/suites/test_suite_debug.function
+++ b/tests/suites/test_suite_debug.function
@@ -48,7 +48,7 @@
     mbedtls_debug_set_threshold( threshold );
     mbedtls_ssl_conf_dbg( &conf, string_debug, &buffer);
 
-    mbedtls_debug_print_msg( &ssl, level, file, line,
+    mbedtls_debug_print_msg_free( &ssl, level, file, line,
                      mbedtls_debug_fmt("Text message, 2 == %d", 2 ) );
 
     TEST_ASSERT( strcmp( buffer.buf, result_str ) == 0 );