chore: set up IBCTestStakingKeeper interface (backport #2028)

[mergify]

This is an automatic backport of pull request #2028 done by Mergify.
Cherry-pick of 4d4dbcf has failed:

On branch mergify/bp/release/v6.0.x/pr-2028
Your branch is up to date with 'origin/release/v6.0.x'.

You are currently cherry-picking commit 4d4dbcf.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   testing/README.md
	modified:   testing/chain_test.go
	new file:   testing/types/expected_keepers.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   testing/app.go
	both modified:   testing/simapp/app.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[damiannolan]

Found this was missing when upgrading icad to v6.0.0-alpha1