Tests: Rename all Principled tests to Principled BSDF

While looking into something else, I noticed that the OSL render tests
were not testing the Principled Volume tests.

This is because when running OSL tests, we skip all Principled BSDF
tests by ignoring all tests that start with `principled_`
(this was done due to noise differences between SVM and OSL).

However this had the knock on effect of skipping tests like
- `principled_absorption`
- `principled_blackbody`
- `principled_smoke`

in the volumetric test suite.

This commit fixes this issue by renaming all Principled BSDF tests to
`principled_bsdf_` and updating the block list to use this new name.

Ref: blender/blender-test-data!31
Pull Request: https://projects.blender.org/blender/blender/pulls/131540

tests/python/cycles_render_tests.py

@@ -62,7 +62,7 @@ BLOCKLIST_OSL = [
     # Noise differences due to Principled BSDF mixing/layering used in some of these scenes
     'render_passes_.*.blend',
     # Noise differences in Principled BSDF mixing/layering
-    'principled_.*.blend',
+    'principled_bsdf_.*.blend',
 ]
 
 BLOCKLIST_OPTIX = [