Improve tests that use `mock/certs.go`

[jackwotherspoon]

We should improve tests that use internal/mock/certs.go

Instead of panic'ing which we currently do it would be better if we were to pass the *testing.T parameter through and fail the test.

cloud-sql-go-connector/internal/mock/certs.go

Lines 126 to 134 in cc63f69

	certDerBytes, err := x509.CreateCertificate(rand.Reader, cert, cert, &k.PublicKey, k)
	if err != nil {
	panic(err)
	}
	c, err := x509.ParseCertificate(certDerBytes)
	if err != nil {
	panic(err)
	}
	return c