Kill off `_PushHTTPChannel`. (#9878)
First of all, a fixup to `FakeChannel` which is needed to make it work with the default HTTP channel implementation. Secondly, it looks like we no longer need `_PushHTTPChannel`, because as of #8013, the producer that gets attached to the `HTTPChannel` is now an `IPushProducer`. This is good, because it means we can remove a whole load of test-specific boilerplate which causes variation between tests and production.
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