Extend a bit the test for dynamic changes to content-visibility, adding
the following checks:
* If there are text descendants in the content-visibility subtree,
make sure they are created when switching to auto ; and removed
when switching back to hidden.
* If there is a content-visibility: hidden or a visibility: hidden
descendant, make sure pruning still applies when switching to
auto.
* If there is a shadow host in the content-visibility subtree,
make sure that shadow subtree is properly updated when
content-visibility changes.
Differential Revision: https://phabricator.services.mozilla.com/D196964
01d8813ff7