Fix select-quoting not preserving newlines in some cases

If the common ancestor of the quoted text is the plain-text-message DIV
itself, the isDescendant test fails and the style to preserve new lines
is not maintained. This adds a non-strict check to isDescendant and
enables that when checking the common ancestor node and a test case for
it.
6 jobs for wip/geary-is-descendant-of in 7 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #202929
fedora

00:03:57

passed #202930
ubuntu

00:02:55

 
  Test
passed #202931
deb-package

00:03:48

canceled #202932
flatpak

00:02:24

 
  Deploy
skipped #202933
review
skipped #202934
allowed to fail manual
stop_review