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/syncthing_test.go

@@ -9,7 +9,6 @@ package syncthing
 import (
 	"io/ioutil"
 	"os"
-	"path/filepath"
 	"testing"
 	"time"
 
@@ -57,13 +56,7 @@ func TestShortIDCheck(t *testing.T) {
 }
 
 func TestStartupFail(t *testing.T) {
-	tmpDir, err := ioutil.TempDir("", "syncthing-TestStartupFail-")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(tmpDir)
-
-	cert, err := tlsutil.NewCertificate(filepath.Join(tmpDir, "cert"), filepath.Join(tmpDir, "key"), "syncthing", 365)
+	cert, err := tlsutil.NewCertificateInMemory("syncthing", 365)
 	if err != nil {
 		t.Fatal(err)
 	}