fixed linux path conversion.

NzbDrone.Test.Common/StringExtensions.cs

@@ -12,6 +12,7 @@ public static string AsOsAgnostic(this string path)
                 if (path.Length > 2 && path[1] == ':')
                 {
                     path = path.Replace(':', Path.DirectorySeparatorChar);
+                    path = Path.DirectorySeparatorChar + path;
                 }
                 path = path.Replace("\\", Path.DirectorySeparatorChar.ToString());
             }