Cycles: Tests: Add option to increase SPP for manual comparisons

Useful for validating changes when sampling/noise changes:
- First run with BLENDER_TEST_UPDATE=1 and e.g. CYCLESTEST_SPP_MULTIPLIER=32
- Apply your change
- Run with only CYCLESTEST_SPP_MULTIPLIER=32
- Compare
- Reset the SVN repo

Differential Revision: https://developer.blender.org/D17107

tests/python/cycles_render_tests.py

@@ -85,6 +85,10 @@ def get_arguments(filepath, output_filepath):
     if custom_args:
         args.extend(shlex.split(custom_args))
 
+    spp_multiplier = os.getenv('CYCLESTEST_SPP_MULTIPLIER')
+    if spp_multiplier:
+        args.extend(["--python-expr", f"import bpy; bpy.context.scene.cycles.samples *= {spp_multiplier}"])
+
     if subject == 'bake':
         args.extend(['--python', os.path.join(basedir, "util", "render_bake.py")])
     elif subject == 'denoise_animation':