Fix safe uint256 sub

[ClementWalter]

Time spent on this PR: 0.1

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

A test is marked as failed while it has the expected behavior
No test for safe sub.

Resolves #683

What is the new behavior?

Fixed tests.
Behavior was already correct.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (806ea6b) 71.84% compared to head (98b8507) 72.11%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #824      +/-   ##
==========================================
+ Coverage   71.84%   72.11%   +0.27%     
==========================================
  Files          47       47              
  Lines        6046     6040       -6     
==========================================
+ Hits         4344     4356      +12     
+ Misses       1702     1684      -18     

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[Eikix]

lgtm