Fix Physics tests for Godot 4 beta 6

[avilches]

It fixes:

• Class name RigidDynamicBody2D to RigidBody2D
• The popup_menu signal "index_pressed" now uses a lambda to fix the binding arguments
• Godot 4 new method names (like deg_to_rad and so on...)
• All HSlider appeared broken. They are fixed using a custom_minimum_size.x value of 50/100.
• Scenes and godot.project files are just updated to Godot 4 beta 6 (I mean, I didn't change anything, Godot updated the values by itself)

All tests now works properly.

[Calinou]

I've updated the same demo in #782, with some cosmetic changes included for better usability.

[avilches]

If the demo is already working in #782, feel free to close this PR or whatever you think is better.

[aaronfranke]

Closing in favor of #782