[docs] Update Testing.md

Author: benrimmington

docs/Testing.md

@@ -24,6 +24,24 @@ We use multiple approaches to test the Swift toolchain.
   locally before committing.  (Usually on a single platform, and not necessarily
   all tests.)
 * Buildbots run all tests, on all supported platforms.
+  [Smoke testing](ContinuousIntegration.md#smoke-testing)
+  skips the iOS, tvOS, and watchOS platforms.
+
+The [test/lit.cfg](https://github.com/apple/swift/blob/master/test/lit.cfg)
+uses an iOS 10.3 simulator configuration named "iPhone 5" for 32-bit testing.
+
+1.  Download and install the iOS 10.3 simulator runtime, in Xcode's
+    [Components](https://help.apple.com/xcode/#/deva7379ae35) preferences.
+
+2.  Create an "iPhone 5" simulator configuration, either in Xcode's
+    [Devices and Simulators](https://help.apple.com/xcode/#/devf225e58da)
+    window, or with the command line:
+
+    ```sh
+    xcrun simctl create 'iPhone 5' 'com.apple.CoreSimulator.SimDeviceType.iPhone-5'
+    ```
+
+3.  Append `--ios` to the `utils/build-script` command line (see below).
 
 ### Testsuite subsets