builtin: use C.fwrite (buffered) for _write_buf_to_fd (#14558)

cmd/tools/modules/testing/common.v

@@ -123,6 +123,7 @@ pub fn (mut ts TestSession) print_messages() {
 			// progress mode, the last line is rewritten many times:
 			if is_ok && !ts.silent_mode {
 				print('\r$empty\r$msg')
+				flush_stdout()
 			} else {
 				// the last \n is needed, so SKIP/FAIL messages
 				// will not get overwritten by the OK ones