Use Fabric builds in iOS tests (#30639)

Summary:
Pull Request resolved: #30639

# Changelog:
[Internal] Enable Fabric builds in iOS tests on Circle CI and Sandcastle.

Reviewed By: fkgozali

Differential Revision: D25700257

fbshipit-source-id: a250dbc9904efec9ded130912a993638f0992373

Author: hramos

.circleci/config.yml

@@ -423,7 +423,7 @@ 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