tests: remove dead code and add a test case

[stropitek]

It seemed like untested code was trying to read a secondary optional part when the document number was already spreading to the optional part.

However, the code could not work because the condition on the check digit being < was not done on the check digit but another longer range of characters.

I haven't found anywhere that the case is even possible in an authentic document, so I removed it.

The test case added is similar to another example, but I liked that it comes from PRADO so I added it anyway. It already passed before the change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.21%. Comparing base (f716198) to head (dd6da8f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   88.66%   89.21%   +0.54%     
==========================================
  Files          48       48              
  Lines         450      445       -5     
  Branches      123      121       -2     
==========================================
- Hits          399      397       -2     
+ Misses         51       48       -3     

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