merge_pr_50135

Add a couple of reftests for the :open pseudo-class.

Differential Revision: https://phabricator.services.mozilla.com/D234331

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1936113
gecko-commit: 6d39e75391fa69f3acc997d45dbf5c56bfe4df77
gecko-reviewers: dshin