Separate fake AMQP channel from real AMQP client

[jerith]

We have a few tests that look for fake AQMP things on the channel objects belonging to clients. As part of the work in #1000, AMQP clients will get real channel objects instead fake ones and will have to look at channel._fake_channel to get at the fake stuff.

To make that transition easier, it seems sensible to wrap the current fake channel with a more realistic one and expose the underlying fake on channel._fake_channel.

[smn]

:+1:

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Reviewed 8 of 8 files at r1. Review status: all files reviewed at latest revision, all discussions resolved.

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[JayH5]

:+1:

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Comments from the review on Reviewable.io

[JayH5]

Reviewed 8 of 8 files at r1. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks pending.

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Comments from the review on Reviewable.io