Step $4$: Implement GroceryList's ExecuteUndo$()$ member function

Implement GroceryList's ExecuteUndo$()$ member function to do the following:

Pop an UndoCommand off the undo stack

Execute the popped undo command

Delete the undo command

File main.cpp has some test functions. Each constructs a GroceryListTest object from a list of string commands. Test $1$ tests code implemented so far: insertion and undo of insertion. Calls to other test functions are commented$-$out in main$().$ Run your code and make sure that test $1$ passes before proceeding.

Step $5$: Implement the SwapCommand class and GroceryList's SwapWithUndo$()$ member function