Test coverage for ABR

[joeyparrish]

Our existing integration tests disable ABR for predictability. We should build tests that cover ABR behavior by simulating network conditions on the client side.

Using the same StreamGenerator class we already use in integration tests, we can avoid unpredictable network conditions. The media data will be loaded into memory at the start of the test suite, and no actual network traffic will be necessary during playback.

That data is still loaded through NetworkingEngine, though, and the test suite can install response filters to force predictable throughput and latency levels that simulate various network conditions.

In that way, we could write test scenarios for ABR to demonstrate that we make appropriate choices and don't trigger a buffering event if it can be avoided.