Fix flaky FlutterChannelsTests, stop mocking NSObject

[jmagman]

I don't know why this is only flaking on arm64 Macs, but instead of mocking out NSObject which seems to continue to be mocked on some background thread, instead mock out the more-specific FlutterTaskQueue protocol. My suspicion is that the no class for metaclass is complaining when it tries to get the isa off an object's metaclass NSObject but the whole NSObject class is mocked out at that point? Something like that.

Turn the tests back on.

Fixes flutter/flutter#126013
Reverts #41740

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[dnfield]

RSLGTM

[dnfield]

This passed on its first attempt on arm64: https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20Engine%20Drone/816996/overview

[hellohuanlin]

Ah I remember I had similar issue before. Mocking common objects like NSObject messes up with XCTest code because the XCTest implementation likely also use it. Though it was not architecture dependent in my case.