[wdspec] Update test network/add_intercept/contexts.py to use new tab for each test
Using a new_tab context for each and every test in the file should avoid side effects between
tests.
Differential Revision: https://phabricator.services.mozilla.com/D224118
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1921767
gecko-commit: 0d6041f18ea1205cc0dbd9cb2fc54bb35ef66f44
gecko-reviewers: webdriver-reviewers, whimboo