[TestBug] Call ModuleLayer.Controller::enableNativeAccess directly rather than via reflection

JDK-8339517 updated our tests to run the Java launcher with `--enable-native-access` when running tests to avoid the native access warnings that result from JEP 472. See JDK-8331671.

We have two tests that dynamically load the JavaFX modules at runtime using a ModuleLayer. Those tests need to call ModuleLayer.Controller::enableNativeAccess to enable native access for the JavaFX modules. This method is a preview feature in JDK 21, so cannot be called directly if we happen to build and test JavaFX with JDK 21. As such, when I added the calls to enableNativeAccess as part of JDK-8339517, I did so using reflection.

Once we bump the minimum JDK to 22, the reflection calls should be replaced with direct calls to enableNativeAccess. See JDK-8340003.