chore(test-specs): avoid silent mistakes when writing tests

[fselmo]

Description

On a review of #3486, I asked one of the Claude agents to sweep the related framework for gotchas that could lead to silent fill for the wrong reasons. It came back with some hits but fell our of scope for that PR, so I'm adding these here with unit tests. This is also important for DevEx but it's mostly important as a security so we don't end up with footguns that can fill tests but end up with unintended JSON fixtures because of silent overrides of certain things the test itself didn't mean to create.

This shouldn't change anything related to the current state of the tests but I did run hasher compare on a fill of eip7928 module at the current forks/amsterdam and on this branch, just as a sanity check, and it's exact.

Checklist

• Ran fast static checks to avoid CI fails, see Code Standards & Verifying Changes: just static
• PR title has the form <type>(<area>): <title>, where <type> and <area> come from an appropriate C-<type>, respectively A-<area>, label. The title should match the target squash commit message.

Cute Animal Picture

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.01%. Comparing base (0d9d919) to head (5e8b801).
⚠️ Report is 1 commits behind head on forks/amsterdam.

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #3510   +/-   ##
================================================
  Coverage            94.01%   94.01%           
================================================
  Files                  624      624           
  Lines                36905    36905           
  Branches              3326     3326           
================================================
  Hits                 34695    34695           
  Misses                1533     1533           
  Partials               677      677           

Flag	Coverage Δ
unittests	94.01% <ø> (ø)

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[marioevz]

Awesome fixes! Just had one request. Approving for now so we can merge once implemented.