feat: add more tests to increase coverage

internal/cert/root_ca.go

@@ -8,6 +8,7 @@ import (
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"encoding/pem"
+	"errors"
 	"fmt"
 	"time"
 
@@ -52,6 +53,9 @@ func (r *rootCA) WithClientCert(config *client.ClientCertConfig) (client.ClientC
 
 // Create a self-signed certificate.
 func newRootCA(config *ca.CaConfig) ([]byte, []byte, error) {
+	if config == nil {
+		return nil, nil, errors.New("ca.CaConfig config is nil")
+	}
 	priv, err := newPrivateKey()
 	if err != nil {
 		return nil, nil, fmt.Errorf("newPrivateKey: %s", err)