test(kuma-cp): fix wait for goroutine to be done (backport #5638)

[mergify]

This is an automatic backport of pull request #5638 done by Mergify.
Cherry-pick of 31ae2bd has failed:

On branch mergify/bp/release-1.7/pr-5638
Your branch is up to date with 'origin/release-1.7'.

You are currently cherry-picking commit 31ae2bdde.
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
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	both modified:   pkg/core/resources/manager/cache_test.go

no changes added to commit (use "git add" and/or "git commit -a")

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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