Make printing/single-line-must-not-be-split-[...].html actually "print".

At some point [1] it ceased to be a printing test, so it didn't really
test anything useful. Make sure that we enter print mode, and turn it
into a PNG test. Reduce the number of lines and use the Ahem font, to
keep the PNG file sizes low.

Mark the test as failing with LayoutNGBlockFragmentation. It used to
pass because there was no block fragmentation involved in the test.

[1] df18d7ad53954822a3b04ca517b7dc14c2f36185

Change-Id: I5a17d76b53515975c90020bd37639f0af4563130
Reviewed-on: https://chromium-review.googlesource.com/c/1371868
Reviewed-by: Aleks Totic <atotic@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615976}
12 files changed