Use arm64 engine variant on simulators in iOS unit tests (#39213)

testing/ios/IosUnitTests/Tests/FlutterEngineConfig.xcconfig

diff --git a/testing/ios/IosUnitTests/Tests/FlutterEngineConfig.xcconfig b/testing/ios/IosUnitTests/Tests/FlutterEngineConfig.xcconfig
index 8d3437a..fd5201b 100644
--- a/testing/ios/IosUnitTests/Tests/FlutterEngineConfig.xcconfig
+++ b/testing/ios/IosUnitTests/Tests/FlutterEngineConfig.xcconfig
@@ -1 +1,2 @@
-FLUTTER_ENGINE=ios_debug_sim_unopt
+FLUTTER_ENGINE[arch=x86_64]=ios_debug_sim_unopt
+FLUTTER_ENGINE[arch=arm64]=ios_debug_sim_unopt_arm64