Add missing adapter tests
Add missing adapter tests

Summary:
Couple more unit tests missing from `IGListAdapter`.

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/287

Differential Revision: D4281952

Pulled By: jessesquires

fbshipit-source-id: da01d22b291071e5b2a7623b263574738456b8b8

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