bachinger 1760d63fc4 Add FakeMultiPeriodLiveTimeline and test case
This timeline will be used in unit test cases of follow-up
CLs. It basically can be used to emulate the timeline created by a
multi-period live media source when the real time advances.

PiperOrigin-RevId: 512665552
2023-03-01 17:19:57 +00:00
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