cmd/syncthing, lib/logger: Set debug flags on SetDebug for tests

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4187

cmd/syncthing/main.go

@@ -269,7 +269,7 @@ func defaultRuntimeOptions() RuntimeOptions {
 	}
 
 	if os.Getenv("STTRACE") != "" {
-		options.logFlags = log.Ltime | log.Ldate | log.Lmicroseconds | log.Lshortfile
+		options.logFlags = logger.DebugFlags
 	}
 
 	if runtime.GOOS != "windows" {