test: fix TODOs in rpc unit tests

[skosito]

Description

Fix TODOs in ethermint rpc unit tests.

Closes: #2302

Type of change

Unit tests fixes.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Include instructions and any relevant details so others can reproduce.

• Tested CCTX in localnet
• Tested in development environment
• Go unit tests
• Go integration tests
• Tested via GitHub Actions

Checklist:

• I have added unit tests that prove my fix feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.54%. Comparing base (c667168) to head (2c39d46).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2329      +/-   ##
===========================================
+ Coverage    68.37%   68.54%   +0.17%     
===========================================
  Files          302      302              
  Lines        19178    19178              
===========================================
+ Hits         13112    13145      +33     
+ Misses        5426     5383      -43     
- Partials       640      650      +10     

Files	Coverage Δ
rpc/backend/call_tx.go	60.26% <ø> (ø)

... and 4 files with indirect coverage changes

[lumtis]

LGTM