operator/pkg/certs: test certs

In this commit, we introduce unit tests for operator `certs` package.
The test cases include but not limited to:

- Test private key generation.
- Test self-signed certificate authority (CA) generation.
- Verify creation of self-signed certificate and key files within the CA.
- Include tests for utility functions, such as appending to Subject Alternative Names in SSL/TLS certificates.
- Add tests for Karmada Store operations related to certificates stored in Kubernetes Secrets:
  - Create
  - Read
  - Update
  - Load

Signed-off-by: Mohamed Awnallah <[email protected]>