[Perl/perl5] d3faea: APItest.xs: flush output in test_bool_internals_func

[Leon Timmermans via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: d3faea291a338406bbae0e506d519f8ea3fa600e
      
https://github.com/Perl/perl5/commit/d3faea291a338406bbae0e506d519f8ea3fa600e
  Author: Leon Timmermans <faw...@gmail.com>
  Date:   2022-05-30 (Mon, 30 May 2022)

  Changed paths:
    M ext/XS-APItest/APItest.xs

  Log Message:
  -----------
  APItest.xs: flush output in test_bool_internals_func

With the explicit flush this test will fail when PERLIO=stdio, because
stdio defaults to block-based buffering on pipes and therefore the
output will come out wrong.