Replace usage of mockall with explicit TestReportBuilder

[Swatinem]

The new TestReportBuilder collects all of the input fed into it. This way one can run assertions on the collected inputs, instead of asserting functions being called.

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This might be a bit controversial, as it is a personal preference to avoid mocks.
The practical advantage is that avoiding mockall brings us one step closer to switch back to stable Rust vs using nightly.

[codecov]

Codecov Report

Attention: Patch coverage is 85.88235% with 48 lines in your changes missing coverage. Please review.

Project coverage is 98.62%. Comparing base (5fed79a) to head (6867663).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Files	Patch %	Lines
src/report/test.rs	55.55%	48 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   99.25%   98.62%   -0.64%     
==========================================
  Files          17       17              
  Lines        7425     7393      -32     
==========================================
- Hits         7370     7291      -79     
- Misses         55      102      +47     

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[matt-codecov]

This might be a bit controversial, as it is a personal preference to avoid mocks.

not a preference i share, haha. but i am not finding mocking in rust to be as easy as it is in any other language i've written tests in, so i guess i'll start trying to design away a need for them in future code