Tweak APItest for new semantics of printf.

Tweak APItest for new semantics of printf.
Now printf() is always real printf() and not #define'd to
PerlIO_stdoutf() so PerlIO_flush() when STDOUT is redirected
back to original file does not affect stdout unless we are
using :stdio as implementation. So things printf()ed XS side
are not written to file (and turn up at end of test).
So add an XS function which does fflush(stdout), modify .pm
file to export it and .t file to call it.

p4raw-id: //depot/perlio@18547