test: remove direct testify dependency

[PozziTiv4ik]

Summary

• replace the suite startup assertion with the standalone Gomega API already used by the test suite
• remove github.com/stretchr/testify as a direct dependency and tidy the module files
• retain testify only as an indirect dependency required by Kubernetes test packages

Closes #143

Testing

• go mod verify
• go build ./...
• go vet ./...
• go test ./... -run '^$'
• go test ./api/... ./internal/fieldpath ./internal/kcp ./internal/webhook

Environment limitation

go test ./internal/controller reaches the new assertion but cannot complete on Windows because envtest expects the kcp server binary and kcp does not publish that server for Windows. The repository CI runs the kcp test matrix on Ubuntu.

[coderabbitai]

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[BergCyrill]

Thank you for picking this up @PozziTiv4ik.
There is a blocker for now: This repository requires signed commits on main. We haven't written it down anywhere (but it should be visible in the PR) - that's on us and it will go into a contributing guideline shortly. We will also move to an assigned-issue-contribution approach then.
For now please amend your commit with a valid signing key.

If signing isn't something you are currently able to setup that is no problem at all - just close the PR and the issue can be assigned to someone else.