Add more FX Robot API tests to tests/system/src/test/java/test/robot/javafx/scene/RobotTest.java

Specifically, each method should be tested with various parameters -- ideally, all possible combinations of various modes of operations (boolean and enum params) and a few different values for x, y, width, height, etc. We should also have tests for documented exceptions (e.g., threading and permission checks, size checks).

We should have another test for screen capture using an asymmetrical image, so that it would fail if the image were right-left or top-bottom reversed.

Add some tests for multi-screen configuration (especially screen capture)

Make sure that screen capture works for multiple screens with different DPIs.