[issue-#1018] EVM Module keeper - Improve code coverage

[GAtom22]

Closes: #1018

Description

Currently, code coverage for the EVM module keeper is about 72%.

The objective of this PR is to bump it to >90%

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #1396 (1a0d732) into main (157f188) will increase coverage by 1.60%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1396      +/-   ##
==========================================
+ Coverage   56.05%   57.66%   +1.60%     
==========================================
  Files         108      108              
  Lines       10074    10074              
==========================================
+ Hits         5647     5809     +162     
+ Misses       4143     4019     -124     
+ Partials      284      246      -38     

Impacted Files	Coverage Δ
x/evm/keeper/msg_server.go	93.61% <0.00%> (+3.19%)	⬆️
x/evm/keeper/state_transition.go	82.55% <0.00%> (+5.52%)	⬆️
x/evm/keeper/statedb.go	81.37% <0.00%> (+8.96%)	⬆️
x/evm/keeper/keeper.go	89.47% <0.00%> (+11.11%)	⬆️
x/evm/keeper/utils.go	82.50% <0.00%> (+15.00%)	⬆️
x/evm/keeper/grpc_query.go	89.20% <0.00%> (+20.14%)	⬆️
x/evm/keeper/migrations.go	100.00% <0.00%> (+50.00%)	⬆️
x/evm/keeper/abci.go	100.00% <0.00%> (+80.00%)	⬆️

[fedekunze]

These tests are great! left some minor comments. Thanks @GAtom22!