x/gov/simulation/proposals.go: add unit tests

[dauTT]

Description

closes a sub-task related to issue #5007

---

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 #6737 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6737      +/-   ##
==========================================
+ Coverage   59.66%   59.85%   +0.18%     
==========================================
  Files         418      512      +94     
  Lines       25070    31303    +6233     
==========================================
+ Hits        14959    18737    +3778     
- Misses       8867    11143    +2276     
- Partials     1244     1423     +179     

[alessio]

Good to go