refactor: use channels on TestableActor to be able to verify sent messages

[TypeLevelConsoli]

TestableActors uses an Arc<Notify> to know when their messages are triggered, but that only notifies a mute signal, no context or message is gathered. By changing it to mpsc::channel instead, we can receive the message passed and compare it to that we sent.

[coveralls]

Pull Request Test Coverage Report for Build 13701998814

Details

• 33 of 33 (100.0%) changed or added relevant lines in 5 files are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.05%) to 75.969%

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Files with Coverage Reduction	New Missed Lines	%
crates/tap-agent/src/agent/sender_account.rs	9	75.74%

Totals
Change from base Build 13624709983:	0.05%
Covered Lines:	8425
Relevant Lines:	11090

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💛 - Coveralls

[suchapalaver]

Really clear solution, I appreciated how this made the tests themselves more readable 👍