[EVM] add increase/decrease approval test for cw20->erc20

[stevenlanders]

Describe your changes and provide context

• adds test for adding/decreasing approval

Testing performed to validate your change

• new test

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.89%. Comparing base (cbe2957) to head (ba8d7ee).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1584      +/-   ##
==========================================
- Coverage   60.94%   60.89%   -0.05%     
==========================================
  Files         366      366              
  Lines       26020    26043      +23     
==========================================
+ Hits        15858    15860       +2     
- Misses       9159     9179      +20     
- Partials     1003     1004       +1     

see 2 files with indirect coverage changes