iOS: Add test for engineAllowHeadlessExecution (#191635)

`FlutterViewController.engineAllowHeadlessExecution` drives whether an
implicitly-created `FlutterEngine` is createed with
`allowHeadlessExecution`, which causes the engine to tear down its shell
when its view controller deallocs. We didn't previously have any test
coverage for this.

The property was added in flutter/engine#19458 with the intent of being
configured from a XIB, and since it's read-only, Interface Builder's
User Defined Runtime Attributes reach it by setting the synthesized ivar
through key-value coding. Declaring a getter, or marking it `@dynamic`,
would leave storyboards unable to set it, which would break it for
end-users.

We now check:
* that the property can be set via `setValue:forKey:` (check KV support)
* that an implicitly-created engine picks it up

I added `allowHeadlessExecution` from the extension in FlutterEngine.mm
to the FlutterEngine (Test) category so we can check it in tests.

Issue: https://github.com/flutter/flutter/issues/112232

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