merge_pr_63019

[wdspec] Add a test for existing iframes for the "prefers-color-scheme" feature and for an invalid feature name in the "emulation.setMediaFeaturesOverride" command.

Differential Revision: https://phabricator.services.mozilla.com/D328782

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2064354
gecko-commit: 534bab6edfac769c594dff05ac0543570655ec9f
gecko-commit-git: 1de82b849a7963faf473e7f7115ce764847a1aa0
gecko-reviewers: webdriver-reviewers, jdescottes