Add Test Case for Repository Errors in Certs Service

[JeffMboya]

Is your feature request related to a problem? Please describe.

There are no test cases that cover repository errors in the Certs Service. This is a potential gap in our test coverage.

Describe the feature you are requesting, as well as the possible use case(s) for it.

Add a test case that simulates repository errors in the Certs Service.

Indicate the importance of this feature to you.

Must-have

Anything else?

Something like this

			desc:    "failed to save in repo",
			token:   token,
			thingID: "2",
			ttl:     ttl,
			pki: pki.Cert{
				ClientCert:     "",
				IssuingCA:      "",
				CAChain:        []string{},
				ClientKey:      "",
				PrivateKeyType: "",
				Serial:         "",
				Expire:         0,
			},
			identifyRes: &magistrala.IdentityRes{Id: validID},
			repoErr:     certs.ErrMissingCerts,
			err:         certs.ErrMissingCerts,
		},