Add Unit test for InputEventMouse

[ARez2]

part of https://github.com/godotengine/godot/issues/43440

Includes tests for:

• InputEventMouse.set_button_mask() and InputEventMouse.get_button_mask()
• InputEventMouse.set_position() and InputEventMouse.get_position()
• InputEventMouse.set_global_position() and InputEventMouse.get_global_position()

I hope those checks are sufficient, if not please let me know or suggest changes.