#deselectItem() should have no effects if attempted on an item that is not already selected
Add some basic tests for multiselect
MultiSelect should have an empty selected array on initialization
selectItem() calls select() on the item
selectItem() should call onSelectItem if defined
selectItem adds the item to the selected array
MultiSelect#selectItem() should not try calling #deselect()
refactored test for multiple selects into 'previously selected' section of tests
test multiselect#selectItem() is basically inert when selecting an item previously selected