Add a WPT for removing a sibling of sticky positioned element.
Bug 1748891 fixed this bug. Add a WPT testing scenario of the reporter's
testcase.
Differential Revision: https://phabricator.services.mozilla.com/D208692
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1612561
gecko-commit: 303f0de5e7a85a64a5351d509c96472baeb7cabd
gecko-reviewers: layout-reviewers, emilio