Add unit tests for #1211

[kfranqueiro]

We've addressed the issues in #1211, and verified it manually, but given the complexity involved, it'd definitely be a good idea to have unit tests for the following:

• check _editorCellListeners after invoking refreshCell (I'd suggest testing with Pagination so you can test with mixing in Pagination and Editor in either order to ensure inheritance works both ways)
• check _editorCellListeners after modifying a store item that is within view
• check _editorCellListeners after scrolling down in a long OnDemandList (set bodyNode.scrollTop) or after calling gotoPage in Pagination