These reftests are copied from flexbox-unbreakable-child-1-*.html and
flexbox-unbreakable-child-1-*-wrap.html with "box-decoration-break:
clone" added to the flex container.
Note that flexbox-unbreakable-child-3-ref.html is redesigned. It is not
copied from flexbox-unbreakable-child-1-ref.html with
"box-decoration-break: clone" added because the bottom border doesn't
show up in sub-test 3 & 4. (bug 1564726 perhaps?)
Differential Revision: https://phabricator.services.mozilla.com/D69475
