[Bug fix] ray_intersect_triangle in python returned None

[Poupine]

Description

Fixed bug where a return statement was missing in src/render/python/shape_v.cpp. Also added a test to cover the function.

Testing

Added a simple unit test to ensure that a single ray traced towards a single triangle intersects it as expected.

Checklist

• My code follows the style guidelines of this project
• My changes generate no new warnings
• My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• I have commented my code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I cleaned the commit history and removed any "Merge" commits
• I give permission that the Mitsuba 3 project may redistribute my contributions under the terms of its license