Halo: Add CallbackMock class.
Halo: Add CallbackMock class.

This helps tests that test functions working with callback functions.

pprkut/lunr,tardypad/lunr,tardypad/lunr,M2Mobi/lunr,M2Mobi/lunr,pprkut/lunr,tardypad/lunr,pprkut/lunr,M2Mobi/lunr