Use Fabric builds in iOS tests

Summary: Enable Fabric builds in iOS tests on Circle CI and Sandcastle.

Differential Revision: D25700257

fbshipit-source-id: a9171c51cc155377d434a5120a7c5c4eeb3a3e27

Author: hramos

.circleci/config.yml

@@ -428,7 +428,7 @@ jobs:
           steps:
             - run:
                 name: Generate RNTesterPods Workspace
-                command: cd packages/rn-tester && bundle exec pod install --verbose
+                command: cd packages/rn-tester && USE_FABRIC=1 bundle exec pod install --verbose
 
       # -------------------------
       # Runs iOS unit tests