Fix MetaList tests, were totally broken

They worked fine with just one group, but once Rob added a second group
everything came falling down.

* Index the return value of findItem() into the correct array
  (list.items for all, list.groups[groupname] for groups)
* Expect null if there is something at the coordinates but it's in the
  wrong group
* Keep track of the next index and expect that for forInsertion when
  findItem() returned null, as well as at the end of each offset

Change-Id: I76438f583cea5694ce04fa2f4e7e88f8f8f236d1
Merge "Fix MetaList tests, were totally broken"