Add test for LoginCommand; create AuthMe mock test util

Had to create a getter for the Management instance in the AuthMe class for mocking, but fields should generally not be accessed globally. Hopefully soon we will be able to make the field private.

src/main/java/fr/xephi/authme/command/executable/login/LoginCommand.java

@@ -24,7 +24,7 @@ public class LoginCommand extends ExecutableCommand {
         final String playerPass = commandArguments.get(0);
 
         // Log the player in
-        plugin.management.performLogin(player, playerPass, false);
+        plugin.getManagement().performLogin(player, playerPass, false);
         return true;
     }
 }