quilt: fix intermittent failure in faildiff.test

This test assumes that if a child process writes one line to stderr and
then another line to stdout, and stderr is redirected to stdout, that
the order the lines will be read is stable.

This isn't the case and occasionally the lines will be read in a
different order.  Change the test to ignore line ordering.

[ YOCTO #14469 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>