Impove test coverage of x/evm/statedb package

[yihuang]

Description

Closes: #876

• coverage: 99.3% of statements

The only two lines of code that is not covered are unreachable practically I believe:

• https://github.com/tharsis/ethermint/blob/main/x/evm/statedb/access_list.go#L116, access list journals are always in correct order.
• https://github.com/tharsis/ethermint/blob/main/x/evm/statedb/statedb.go#L292, len(value) is always positive.

---

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

---

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)

[fedekunze]

LGTM 💯

[codecov]

Codecov Report

Merging #888 (3de7452) into main (4320f46) will increase coverage by 3.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #888      +/-   ##
==========================================
+ Coverage   56.86%   60.04%   +3.18%     
==========================================
  Files          77       77              
  Lines        6215     6195      -20     
==========================================
+ Hits         3534     3720     +186     
+ Misses       2487     2293     -194     
+ Partials      194      182      -12     

Impacted Files	Coverage Δ
x/evm/statedb/access_list.go	96.42% <ø> (+90.63%)	⬆️
x/evm/statedb/state_object.go	100.00% <ø> (+21.48%)	⬆️
x/evm/statedb/statedb.go	99.20% <ø> (+32.41%)	⬆️
x/evm/statedb/journal.go	100.00% <0.00%> (+28.88%)	⬆️
x/evm/statedb/config.go	100.00% <0.00%> (+50.00%)	⬆️

[yihuang]

#536 (comment)

it seems the non-deterministic issue still exists, we should reopen that issue.