[wdspec] Use new tabs for each bidi fetchError test
Using a dedicated tab for each test should avoid side effects between tests.
Differential Revision: https://phabricator.services.mozilla.com/D217870
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1910126
gecko-commit: bb3ee1769df45266fc18f5c089d7cbc3fafc3494
gecko-reviewers: webdriver-reviewers, Sasha