iOS: Improve FlutterEngineGroup tests (#191563)

Adds tests for `[FlutterEngine spawn]` that do checks at the
`flutter::Shell` level.

`testSpawn` already verified that a spawned engine shares its spawner's
thread host this adds tests that ensure that spawned engines share a
single Dart VM, shared task runners, and that the GPU enabled/disabled
state is correctly set on the underlying `Shell`, not just on the
`FlutterEngine` itself.

This fills in some test coverage gaps before we start moving iOS towards
the embedder API.

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

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