Fix G115 false positive when going from parsed uint to larger int

[hairyhenderson]

Fixes the issue I recently found in #1212 (comment).

When converting to an int* from a uint64 created by strconv.ParseUint with a specified bit size less than the size of the destination integer, currently G115 is falsely triggered.

That's because the current check only allows the source and destination sign to match (i.e. from int* to int* or uint* to uint*). This is a useful check when going from ParseInt to a uint*, but when going from ParseUint to an int* the conversion is safe as long as the bit size is at least one less.

I've added the extra test and documented it since the logic is somewhat complex.

I've also added a few test cases to catch regressions.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.71%. Comparing base (1216c9b) to head (78e7ae6).
Report is 9 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1254      +/-   ##
==========================================
- Coverage   68.49%   66.71%   -1.79%     
==========================================
  Files          75       75              
  Lines        4384     5195     +811     
==========================================
+ Hits         3003     3466     +463     
- Misses       1233     1581     +348     
  Partials      148      148              

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