test(json): improve json testing

[kt3k]

This PR improves the test cases of json. With this change, this change makes the line coverage of std/json 100%.

part of #3713

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.42%. Comparing base (16fa0c8) to head (c429764).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5075      +/-   ##
==========================================
+ Coverage   92.41%   92.42%   +0.01%     
==========================================
  Files         481      481              
  Lines       38752    38749       -3     
  Branches     5449     5449              
==========================================
+ Hits        35811    35814       +3     
+ Misses       2886     2880       -6     
  Partials       55       55              

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