Updated offers tests to be less flaky

[9larsons]

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• Offers browser tests have been occasionally flaky. The error message is not particularly helpful, saying that the test generically timed out. The offers iframe attachment has been changed to an assertion and updated to separate locator and toBeAttached calls to better determine the point of failure.
• Reruns were bumping up against the 60s test timeout; this has been extended to 90s to help reruns finish in the event they need to run.