Unit test -[IGListAdapter scrollToItem:...]
Unit test -[IGListAdapter scrollToItem:...]

Summary:
- [x] Vertical/horizontal scrolling
- [x] No supplementary view
- [x] Single supplementary view
- [x] Multiple supplementary views
- [x] Item not in feed
- [x] Empty feed
Closes https://github.com/Instagram/IGListKit/pull/179

Differential Revision: D4166911

Pulled By: rnystrom

fbshipit-source-id: 08a0fcf8259df97265b8938248e08f7a23a38a0e

zhubofei/IGListKit,Sherlouk/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,charles0126dev/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,dshahidehpour/IGListKit,charles0126dev/IGListKit,charles0126dev/IGListKit,dshahidehpour/IGListKit,dshahidehpour/IGListKit,Sherlouk/IGListKit,Instagram/IGListKit,Sherlouk/IGListKit,zhubofei/IGListKit