[wdspec] Improve "browsingContext.setViewport" tests to trigger reflows.

Without an actual page loaded, changing the viewport will not trigger any reflow and as such doesn't test real use-case scenarios.

Differential Revision: https://phabricator.services.mozilla.com/D194624

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1865618 gecko-commit: 988211aadb0d43726c920e868dfa7dd055f7fa77 gecko-reviewers: webdriver-reviewers, Sasha