Add BFC tests for (-webkit-)line-clamp (#46734)
As was recently resolved in w3c/csswg-drafts#10323 and
w3c/csswg-drafts#10324, (-webkit-)line-clamp should always create an
independent formatting context whenever it applies. Furthermore, the
display: -webkit-box property will not cause the box to become a
flexbox, but it will be an independent block formatting context
instead (even for line-clamp, which doesn't require
display: -webkit-box to apply).
This patch adds some tests for it, and makes some other tests no
longer tentative.
Co-authored-by: Florian Rivoal git@florian.rivoal.net