[wdspec] Add tests for initiatorType and destination for iframe loads
Depends on D230175
Add new test request in initiatorType / destination tests to check iframe document loads.
Differential Revision: https://phabricator.services.mozilla.com/D230178
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1933331
gecko-commit: 04f9bd3ee9b6f304ce381a28a89f3476b6ec04d8
gecko-reviewers: webdriver-reviewers, Sasha