lib/syncthing: Clean up / refactor LoadOrGenerateCertificate() utility function. (#8025)

LoadOrGenerateCertificate() takes two file path arguments, but then
uses the locations package to determine the actual path.  Fix that
with a minimally invasive change, by using the arguments instead.
Factor out GenerateCertificate().

The only caller of this function is cmd/syncthing, which passes the
same values, so this is technically a no-op.

* lib/tlsutil: Make storing generated certificate optional.  Avoid
  temporary cert and key files in tests, keep cert in memory.

lib/syncthing/utils.go

@@ -25,22 +25,18 @@ import (
 )
 
 func LoadOrGenerateCertificate(certFile, keyFile string) (tls.Certificate, error) {
-	cert, err := tls.LoadX509KeyPair(
-		locations.Get(locations.CertFile),
-		locations.Get(locations.KeyFile),
-	)
+	cert, err := tls.LoadX509KeyPair(certFile, keyFile)
 	if err != nil {
-		l.Infof("Generating ECDSA key and certificate for %s...", tlsDefaultCommonName)
-		return tlsutil.NewCertificate(
-			locations.Get(locations.CertFile),
-			locations.Get(locations.KeyFile),
-			tlsDefaultCommonName,
-			deviceCertLifetimeDays,
-		)
+		return GenerateCertificate(certFile, keyFile)
 	}
 	return cert, nil
 }
 
+func GenerateCertificate(certFile, keyFile string) (tls.Certificate, error) {
+	l.Infof("Generating ECDSA key and certificate for %s...", tlsDefaultCommonName)
+	return tlsutil.NewCertificate(certFile, keyFile, tlsDefaultCommonName, deviceCertLifetimeDays)
+}
+
 func DefaultConfig(path string, myID protocol.DeviceID, evLogger events.Logger, noDefaultFolder bool) (config.Wrapper, error) {
 	newCfg, err := config.NewWithFreePorts(myID)
 	if err != nil {