x/distribution: simapp for testing

[jgimeno]

Closes: #4875

Description

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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 #5725 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #5725     +/-   ##
=========================================
+ Coverage   35.66%   35.96%   +0.3%     
=========================================
  Files         337      337             
  Lines       34639    34639             
=========================================
+ Hits        12354    12459    +105     
+ Misses      21056    20934    -122     
- Partials     1229     1246     +17

Impacted Files	Coverage Δ
x/distribution/keeper/hooks.go	30.23% <100%> (ø)	⬆️
x/distribution/keeper/validator.go	87.23% <100%> (ø)	⬆️
x/distribution/keeper/delegation.go	68% <100%> (ø)	⬆️
x/distribution/keeper/querier.go	55.4% <100%> (ø)	⬆️
x/distribution/handler.go	8.82% <0%> (+1.47%)	⬆️
x/distribution/keeper/store.go	50% <0%> (+3.47%)	⬆️
x/distribution/genesis.go	10% <0%> (+10%)	⬆️
x/distribution/module.go	54.09% <0%> (+54.09%)	⬆️
x/distribution/keeper/invariants.go	54.63% <0%> (+54.63%)	⬆️

[tac0turtle]

ci tests should be marked as required.