Make line-clamp tests non-tentative when both alternatives agree
Currently the specification of line-clamp uses a variant called
continue: discard, but there is an alternative called
continue: collapse (see w3c/csswg-drafts#7708), which is being
implemented in Chrome. A number of tests were added to test this
collapse alternative, marked as tentative. This patch makes some of
those tests non-tentative, in cases when both of the alternatives
agree.
The agreement of both variants is according to the spec as it exists
right now, independent of yesterday's CSSWG resolutions. A follow-up
patch will make further tests non-tentative to account for those
resolutions.