Add some basic pagination tests.

Testing various basic things, such as several break-before:page /

combined with break-after:avoid, and also that the implementation
actually supports multiple pages, and doesn't just output the first or
last page.

break-after: page at the same break opportunity, break-before:page
Change-Id: Iae645a98d602f7914c04c58e7ae647ca0c490f8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5831010
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1351698}