Make testing blank state more reliable

When migrating the config file, move it instead of copying it, otherwise
when we test blank state we always migrate the legacy config file in.

desktop/src/com/agateau/pixelwheels/desktop/DesktopLauncher.java

@@ -100,6 +100,6 @@ public class DesktopLauncher {
         }
 
         System.err.printf(Locale.US, "Migrating %s to %s\n", legacyConfigFile, configFile);
-        legacyConfigFile.copyTo(configFile);
+        legacyConfigFile.moveTo(configFile);
     }
 }