types: add test cases

[alessio]

Increase types package coverage by 7%.

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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 #5747 into master will increase coverage by 0.77%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5747      +/-   ##
==========================================
+ Coverage   34.88%   35.66%   +0.77%     
==========================================
  Files         337      337              
  Lines       34647    34639       -8     
==========================================
+ Hits        12087    12354     +267     
+ Misses      21323    21056     -267     
+ Partials     1237     1229       -8

Impacted Files	Coverage Δ
types/address.go	73.47% <100%> (+9.71%)	⬆️
types/uint.go	79.03% <0%> (+4.03%)	⬆️
types/int.go	75.53% <0%> (+5.85%)	⬆️
types/context.go	92.23% <0%> (+21.35%)	⬆️
types/utils.go	75% <0%> (+30%)	⬆️
types/config.go	98.66% <0%> (+33.33%)	⬆️
types/errors/abci.go	96.92% <0%> (+36.92%)	⬆️
types/store.go	75% <0%> (+56.25%)	⬆️
types/tx_msg.go	84.61% <0%> (+84.61%)	⬆️
... and 3 more