Migrate workspace item adding tests to kotlin

In a former CL (http://ag/16064580) I've tried to
do the same but presubmit was constantly failing,
probably because the test is in kotlin.

Test: AddWorkspaceItemsTaskTest.kt
Bug: 199160559
Change-Id: Ie1bc4fcd4f94cd7cb0601c21bbdf273452b9dd1f