[wdspec] Fix continueResponse cookie test intermittent with flaky expiry date parsing
The parsed expiry for cookies might be slightly different from the provided value, because
Firefox will attempt to compensate the delta between the server time and the current time.
Instead of expecting an exact match, assert that the actual expiry is within 1ms of the expected
value.
Differential Revision: https://phabricator.services.mozilla.com/D221266
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1916221
gecko-commit: b04eab8729b75766592b8de69f5a1280ad6a47e0
gecko-reviewers: baku, webdriver-reviewers, Sasha