Tests: Add render tests for outputs of the light path node

This commit adds tests for the outputs of the light path node.

This include many typical tests (E.g. Changing the colour of a material
based on if it's a camera ray or not) along with some "unusual" tests.

Examples include:
- Testing many of the light path node outputs on lights.
  - This includes "analytical" lights and mesh lights, which can behave
    differently depending on if Next Event Estimation or forward path
    tracing occurs.
- Adding extra objects using ray portal BSDFs and transparent BSDFs
  just to double check they don't introduce their own issues.

Ref: blender/blender-test-data!55

tests/python/storm_render_tests.py

@@ -39,6 +39,10 @@ BLOCKLIST_METAL = [
     "underwater_caustics.blend",
     "shadow_link_transparency.blend",
     "principled_bsdf_transmission.blend",
+    "light_path_is_shadow_ray.blend",
+    "light_path_is_transmission_ray.blend",
+    "light_path_ray_depth.blend",
+    "light_path_ray_length.blend",
     # Volume
     "light_link_surface_in_volume.blend",
     "openvdb.*.blend",