Parse messages in session tests so assertions can be made on their details

[davidsteiner]

It also removes the separate spawned task for messages received from the session and consumes messages directly from the channel connecting the mock counterparty to the session.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.08%. Comparing base (6e7ea11) to head (91335ae).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/hotfix/tests/common/mock_counterparty.rs	84.61%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   59.95%   60.08%   +0.12%     
==========================================
  Files          66       66              
  Lines        2712     2723      +11     
==========================================
+ Hits         1626     1636      +10     
- Misses       1086     1087       +1     

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