[ios-sdk] Added a unit test for REST API calls
[ios-sdk] Added a unit test for REST API calls

Summary:
We were implicitly testing REST calls because token extension uses it, but did not have a unit test
case that explicitly tested it. Since this was useful to me for Android porting, adding it to iOS as well.

Test Plan:
- Ran the unit tests

Revert Plan:

Reviewers: jacl, mmarucheck, gregschechte, ayden

Reviewed By: mmarucheck

CC: msdkexp@

Differential Revision: https://phabricator.fb.com/D531853

Subsplash/facebook-ios-sdk,taptaptap/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,anthonylai/facebook-ios-sdk,flovilmart/facebook-ios-sdk,anthonylai/facebook-ios-sdk,Citrrus/facebook-ios-sdk,taptaptap/facebook-ios-sdk,Citrrus/facebook-ios-sdk,shepting/facebook-ios-sdk,shepting/facebook-ios-sdk,taptaptap/facebook-ios-sdk,anthonylai/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,shepting/facebook-ios-sdk,tipbit/facebook-ios-sdk,wooga/facebook-ios-sdk,1234-/facebook-ios-sdk,shepting/facebook-ios-sdk,flovilmart/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,tipbit/facebook-ios-sdk,hibu/facebook-ios-sdk,beamly/facebook-ios-sdk,1234-/facebook-ios-sdk,beamly/facebook-ios-sdk,hibu/facebook-ios-sdk,Amnysia/facebook-ios-sdk,wooga/facebook-ios-sdk,1234-/facebook-ios-sdk,TimeIncOSS/facebook-ios-sdk,Citrrus/facebook-ios-sdk,ShareKit/facebook-ios-sdk,flovilmart/facebook-ios-sdk,ShareKit/facebook-ios-sdk,Subsplash/facebook-ios-sdk,hibu/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,flovilmart/facebook-ios-sdk,shepting/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,FiftyThree/facebook-ios-sdk,ArtLapitski/facebook-ios-sdk,1234-/facebook-ios-sdk,hibu/facebook-ios-sdk,Amnysia/facebook-ios-sdk,Amnysia/facebook-ios-sdk,wooga/facebook-ios-sdk,flovilmart/facebook-ios-sdk,hibu/facebook-ios-sdk,tipbit/facebook-ios-sdk,1234-/facebook-ios-sdk,ShareKit/facebook-ios-sdk,Subsplash/facebook-ios-sdk,beamly/facebook-ios-sdk