Test for section controller retain cycles
Test for section controller retain cycles

Summary:
Saw our [retain cycle detector](https://github.com/facebook/FBRetainCycleDetector) fire for the `_objectTransitionBlock` in the adapter (t15355965) so I added a test. The test passes, so I don't think that is what's going on, but figured I'd submit the test anyways.

- [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/401

Differential Revision: D4398534

Pulled By: rnystrom

fbshipit-source-id: d5f07a1222c28553dbac66971a90731b2f1c397f

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