*: more unit tests for testing SSZ

[KaloyanTanev]

As a preparation for the migration from fastssz to dynamic-ssz, those are more unit tests that test all of our usages of the ssz package.

In a subsequent PR will be the actual migration.

category: test
ticket: none

[pinebit]

I love this approach well aligned with Martin Fowler's refactoring science.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.58%. Comparing base (be67ac5) to head (795a35c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4416      +/-   ##
==========================================
+ Coverage   56.42%   56.58%   +0.15%     
==========================================
  Files         244      244              
  Lines       32421    32496      +75     
==========================================
+ Hits        18294    18387      +93     
+ Misses      11779    11768      -11     
+ Partials     2348     2341       -7     

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