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 36.68%. Comparing base (caebdea) to head (ab026d0).

❌ Your project check has failed because the head coverage (36.68%) 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   36.66%   36.68%   +0.02%     
==========================================
  Files        1165     1165              
  Lines      104401   104403       +2     
==========================================
+ Hits        38281    38305      +24     
+ Misses      62870    62848      -22     
  Partials     3250     3250              

Files with missing lines	Coverage Δ
evmrpc/tests/utils.go	69.18% <100.00%> (+0.33%)	⬆️

... and 11 files with indirect coverage changes

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