Added 'setLength' function to Carousel Mask.

A length set with 'setLength' will be preferentially used by 'getLength' instead of the measured length.
Expose 'setMaskLength' on the carousel core.
Improve a unit test
Merge pull request #157 from fmtvp/carousel-culling-mask-length-fix

Reviewed. Test failure on last commit unrelated; TVPJSFRMWK-1986 raised to address it.