[wdpspec] Add tests for "browsingContext.navigationFailed" and "browsingContext.navigationStarted" events for beforeunload case.
Differential Revision: https://phabricator.services.mozilla.com/D217380
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1846601
gecko-commit: 9e71df83ded976d5f80ef67db59c061a0f96efae
gecko-reviewers: webdriver-reviewers, jdescottes