Flush TAP output lines after printing

TAP output is mostly meant to be consumed by something else, but because lua buffers output when piped to another process the consumer wouldn't get output until the whole process had finished (or the buffer was full).

It might be worth looking into a more generic solution to this problem, because the issue could come up with other output formats as well. One possibility is:

``` lua
io.stdout:setvbuf 'no'
```

but I'm not sure what the side effects of that would be, if any.