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.