Fix unit test failure due to lack of string.trim() function

api.lua

@@ -74,7 +74,6 @@ function classroom.get_group(name)
 end
 
 function classroom.create_group(name)
-	name = name:trim()
 	if _groups[name] or #name == 0 then
 		return nil
 	end

gui_group.lua

@@ -19,7 +19,7 @@ minetest.register_on_player_receive_fields(function(player, formname, fields)
 		return false
 	end
 
-	local group = classroom.create_group(fields.name)
+	local group = classroom.create_group(fields.name:trim())
 	if group then
 		sfinv.set_player_inventory_formspec(player)
 		classroom.show_edit_group(player, group.name)