refactor: remove weird casting for staking.Keeper in slashing and distribution

[robert-zaremba]

Description

Both slashing and distribution simulator is using ugly casting from the nice interface type to:

stakeKeeper := sk.(*stakingkeeper.Keeper)

This creates additional dependency for staking/keeper package in other modules, and requires type of a specific type (here reference type, in 0.46 it's even worse - because it uses object type).

EXTRA:

• removed stakingkeepr.RandomValidator function (which btw, was also weird, because it was in the keeper module and required Keeper as an argument) and created generic testutil.RandSliceElem function.

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Author Checklist

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• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
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• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
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I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
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• reviewed state machine logic
• reviewed API design and naming
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• reviewed tests and test coverage
• manually tested (if applicable)

[tac0turtle]

Thanks for the cleanup

[julienrbrt]

lgtm!

[codecov]

Codecov Report

Merging #12973 (b8a021f) into main (4fe7797) will decrease coverage by 0.29%.
The diff coverage is 62.63%.

@@            Coverage Diff             @@
##             main   #12973      +/-   ##
==========================================
- Coverage   55.87%   55.58%   -0.30%     
==========================================
  Files         646      648       +2     
  Lines       54895    54925      +30     
==========================================
- Hits        30675    30532     -143     
- Misses      21762    21929     +167     
- Partials     2458     2464       +6     

Impacted Files	Coverage Δ
baseapp/abci.go	64.09% <0.00%> (-0.33%)	⬇️
client/flags/flags.go	19.35% <0.00%> (-0.32%)	⬇️
client/rpc/status.go	66.66% <ø> (ø)
client/utils.go	34.92% <0.00%> (ø)
server/rosetta/client_online.go	1.35% <0.00%> (ø)
server/swagger.go	0.00% <0.00%> (ø)
simapp/app.go	82.66% <0.00%> (+3.55%)	⬆️
testutil/list.go	0.00% <0.00%> (ø)
types/result.go	76.74% <ø> (ø)
x/auth/tx/query.go	0.00% <ø> (ø)
... and 48 more