Test3D: Change absolute filepaths to relative

[samclane]

Problem:

Running the game Test3D from the launcher gives:

Traceback (most recent call last):
  File "main.py", line 58, in <module>
  File "//Games/TestGames/Test3D/main.py", line 96, in <module>
OSError: [Errno 2] ENOENT

This is probably because when running it via Thonny serial connection, the working directory is /, so the absolute pathing works fine in that case.

Solution:

Change file paths to relative, so Test3D can be run from the launcher.