[Gecko Bug 1880859] [wdspec] Use a new tab for each test in add_intercept/url_patterns.py

[moz-wptsync-bot]

Differential Revision: https://phabricator.services.mozilla.com/D202163

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1880859 gecko-commit: f9f3f34a88396f07c5821dc9304fdc8a096d5054 gecko-reviewers: webdriver-reviewers, whimboo