Add license information

ResticLogParserTests/FileSizeTests.cs

@@ -1,3 +1,18 @@
+// ResticLogParser
+// Copyright (C) 2025  Stefan Müller
+//
+// This program is free software: you can redistribute it and/or modify it under
+// the terms of the GNU General Public License as published by the Free Software
+// Foundation, either version 3 of the License, or (at your option) any later
+// version.
+//
+// This program is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along with
+// this program.  If not, see <http://www.gnu.org/licenses/>.
+
 using ResticLogParser.src;
 
 namespace ResticLogParserTests