Common class names cause naming collisions

[rgson]

The used Godot version:

3.5 stable.

Describe the bug

gdUnit3 classes with common class_names cause naming collisions with users' code.

Loading GdUnit3 Plugin success
 res://src/Fonts.gd:1 - Parse Error: Unique global class "Fonts" already exists at path: res://addons/gdUnit3/src/ui/Fonts.gd

Steps to Reproduce

1. Create a script with class_name Fonts.
2. Install gdUnit3.

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I suggest prepending GdUnit to all class names to avoid collisions. Many classes have this prefix already.