Add RPC testing README

[codchen]

Describe your changes and provide context

Add documentation to facilitate test writing for EVM RPC.

Testing performed to validate your change

n/a

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.79%. Comparing base (a13a275) to head (2be8ae1).
⚠️ Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (47.29%) is below the target coverage (60.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2406       +/-   ##
===========================================
+ Coverage   39.69%   50.79%   +11.09%     
===========================================
  Files        1571     1581       +10     
  Lines      161065   161361      +296     
===========================================
+ Hits        63938    81961    +18023     
+ Misses      92269    73163    -19106     
- Partials     4858     6237     +1379     

Flag	Coverage Δ
sei-chain	27.35% <ø> (+0.01%)	⬆️
sei-cosmos	51.95% <ø> (-0.01%)	⬇️
sei-tendermint	58.65% <ø> (?)
sei-wasmd	51.40% <ø> (-0.03%)	⬇️

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see 181 files with indirect coverage changes

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