Fixes #17103 which has two problems, one is that the checkbox is not displayed in the drop-down, and the second problem is that the initial ComboBox label is not displayed, both are fixed in dojox/form/CheckedMultiSelect.js. Also update the dojox/mvc test related to the change which caused part of this problem to be sure to avoid breaking dojox/mvc.
(cherry picked from commit 1a7a9849a0d17d88854fca05c589dee97aa13353)