Nice. Comprehensive unit tests are indeed welcome. Even a simple test code that tests all features, flags combinations, etc, and outputs a log. It will make easy for anyone to run it and compare the logs.

I had a bug:

    u_printf( my_output_cb, NULL, "%d" NL, -42 );

--42

I may have found an explanation. In u_parse_format(), case 'i': calls u_itoa( value, buffer, 10, false ); which outputs minus sign.

Then if ( number ) does if ( sign ) { output_cb( '-', ctx ); chars_written++; } which outputs another minus sign.