Make Sidekiq Web UI tests less flaky

What does this MR do and why?

This backports !152420 (merged) to 16-11-stable-ee.

One of the Sidekiq Web UI test attempts to test that the latency shown in the queues page is 0.0. However, if there is some slowness on the runner the latency could tick up to a second, causing the test to fail.

Avoid this flakiness by stubbing Time.now so that we guarantee that the latency will always remain at 0.

Relates to #460842 (closed)

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