Increase coverage across the store package

[alessio]

Run make mocks to ensure reproducibility.

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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)

[alexanderbez]

ACK

[codecov]

Codecov Report

Merging #5727 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5727      +/-   ##
==========================================
+ Coverage   35.77%   35.79%   +0.01%     
==========================================
  Files         335      336       +1     
  Lines       32719    32727       +8     
==========================================
+ Hits        11706    11714       +8     
  Misses      19783    19783              
  Partials     1230     1230

Impacted Files	Coverage Δ
server/pruning.go	100% <100%> (ø)