x/mint/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 #6706 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6706   +/-   ##
=======================================
  Coverage   58.54%   58.54%           
=======================================
  Files         509      509           
  Lines       30887    30887           
=======================================
  Hits        18084    18084           
  Misses      11465    11465           
  Partials     1338     1338