test(x/slashing): write integration tests

[likhita-809]

Description

ref: #14145

This PR refactors the slashing integration tests to use new testing framework setup and de-duplicates some code.

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[julienrbrt]

looks good, but like Marko said, I think we should rather have integration tests via msgs. Presently, it isn't clear what is a unit test and what is an integration test for slashing.
I think we should be coherent. Currently, the message server isn't used at all in the rewritten integration tests.

Nice again to see those e2e to be deleted!!

[atheeshp]

lgtm

[julienrbrt]

@likhita-809 could you revert the app.go changes? A PR already fixed it (and there is a conflict here now).