ios: set Fastlane test groups

ios/fastlane/Fastfile

       changelog: ENV["JITSI_CHANGELOG"],
       demo_account_required: false,
       distribute_external: true,
+      groups: ENV["JITSI_BETA_TESTING_GROUPS"],
       reject_build_waiting_for_review: true,
       uses_non_exempt_encryption: false
     )