Ignore unguarded-availability for unit test (flutter#44852)

Ignore ungarded-availability for unit test

fixes flutter/flutter#128958

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

shell/platform/darwin/ios/BUILD.gn

@@ -227,6 +227,12 @@ source_set("ios_test_flutter_mrc") {
     "-F$platform_frameworks_path",
     "-mios-simulator-version-min=$ios_testing_deployment_target",
   ]
+
+  # XCode 15 beta has a bug where iOS 17 API usage is not guarded.
+  # This bug results engine build failure since the engine treats warnings as errors.
+  # The `-Wno-unguarded-availability-new` can be removed when the XCode bug is fixed.
+  # See details in https://github.com/flutter/flutter/issues/128958.
+  cflags_objcc = [ "-Wno-unguarded-availability-new" ]
   sources = [
     "framework/Source/FlutterEnginePlatformViewTest.mm",
     "framework/Source/FlutterEngineTest_mrc.mm",
@@ -271,6 +277,12 @@ shared_library("ios_test_flutter") {
     "-fobjc-arc",
     "-mios-simulator-version-min=$ios_testing_deployment_target",
   ]
+
+  # XCode 15 beta has a bug where iOS 17 API usage is not guarded.
+  # This bug results engine build failure since the engine treats warnings as errors.
+  # The `-Wno-unguarded-availability-new` can be removed when the XCode bug is fixed.
+  # See details in https://github.com/flutter/flutter/issues/128958.
+  cflags_objcc = [ "-Wno-unguarded-availability-new" ]
   ldflags = [
     "-F$platform_frameworks_path",
     "-Wl,-install_name,@rpath/Frameworks/libios_test_flutter.dylib",