Write buffered data to file before reading
Write buffered data to file before reading

On MoarVM we don't buffer the first write. The additional call to 'flush'
makes the test routine independent of that implementation detail.
Cmp. https://irclog.perlgeek.de/perl6-dev/2018-05-14#i_16164630

perl6/roast,dogbert17/roast,dogbert17/roast