CppunitTest_sw_layoutwriter2: use more SwParaPortion::dumpAsXml()
See commit
feeed3e762cf077fbd9cf48f82e949365108ccc1
(CppunitTest_sw_layoutwriter: avoid some a11y-based layout testing,
2022-04-07) for motivation.
Also add a SwBookmarkPortion::dumpAsXml() can that show the colors of
a bookmark portion, instead of faking that into the layout text.
Change-Id: Id4da6b27bf3d33d55bba281445084242306d1525
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/140751
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>