Fix #2431 test failed

When launch from test framework, the Application.StartupPath looks like "C:\PROGRAM FILES (X86)\MICROSOFT VISUAL STUDIO\2019\COMMUNITY\COMMON7\IDE\EXTENSIONS\TESTPLATFORM" which may not have write permission.

shadowsocks-csharp/Util/Util.cs

@@ -38,7 +38,7 @@ namespace Shadowsocks.Util
                 {
                     if (isPortableMode)
                     {
-                        _tempPath = Directory.CreateDirectory(Path.Combine(Application.StartupPath, "ss_win_temp")).FullName;
+                        _tempPath = Directory.CreateDirectory("ss_win_temp").FullName;
                         // don't use "/", it will fail when we call explorer /select xxx/ss_win_temp\xxx.log
                     }
                     else