Summary:
Use Xcode 10.1 and iOS 12.1 SDK to run iOS tests on an iOS 12.1 iPhone XS Simulator, all of which are available on a default Xcode 10.1 install.
Note that we were previously running iOS tests using Xcode 10.1 and the 12.1 SDK in a separate `test_xcode10` workflow on Circle CI. This was in place to allow us to track Xcode 10 compatibility during the Xcode 10 beta. Now that Xcode 10 has been public for a while, we can drop the compatibility check and use Xcode 10 in our main iOS test workflow.
Reviewed By: TheSavior
Differential Revision: D13317891
fbshipit-source-id: 04c17bf3a2e9d3617f14a46b4ed30a5491a4f4a4