test(p/uint25): Increase Test Coverage for `uint256` Package

[notJoon]

Description

Increased the test coverage of the p/demo/uint256 package. Previously, only about 40% was covered, but now it has increased to 90% (checked in go).

The existing implementation of the uint256 function is unmodified except for modifying it to use the strconv package.

• [X] Added new tests, or not needed, or not feasible
• [ ] Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• [ ] Updated the official documentation or not needed
• [ ] No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• [X] Added references to related issues and PRs
• [ ] Provided any useful hints for running manual tests
• [ ] Added new benchmarks to generated graphs, if any. More info here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.39%. Comparing base (247f2c6) to head (d797500). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2931      +/-   ##
==========================================
- Coverage   63.39%   63.39%   -0.01%     
==========================================
  Files         565      565              
  Lines       79390    79390              
==========================================
- Hits        50329    50327       -2     
- Misses      25673    25674       +1     
- Partials     3388     3389       +1     

Flag	Coverage Δ
contribs/gnodev	60.57% <ø> (ø)
contribs/gnofaucet	15.77% <ø> (+0.94%)	:arrow_up:
gno.land	67.37% <ø> (ø)
gnovm	67.86% <ø> (ø)
misc/genstd	79.72% <ø> (ø)
tm2	62.43% <ø> (ø)

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[jefft0]

Removed the "review/triage-pending" label because jefft0 approved. Ready for core dev review.