x/bank/simulation/genesis.go: add unit tests

[dauTT]

Description

closes a sub-task related to issue #5007

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• 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
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #6772 into master will increase coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6772      +/-   ##
==========================================
+ Coverage   59.68%   59.89%   +0.20%     
==========================================
  Files         389      513     +124     
  Lines       25257    31491    +6234     
==========================================
+ Hits        15075    18861    +3786     
- Misses       9060    11192    +2132     
- Partials     1122     1438     +316