Fix: Cycles denoise animation test is flaky with parallel ctest

Write intermediate file in the relevant test directory in the build
folder, rather than next to the blend file, so the OSL and SVM variants
don't interfere.

Started happening after a78585b00d re-renabled this test.

Pull Request: https://projects.blender.org/blender/blender/pulls/156700

tests/files/render/util/render_denoise.py

@@ -13,7 +13,7 @@ def denoise(context):
 
     filepath = bpy.data.filepath
 
-    output_filepath = filepath + ".output.exr"
+    output_filepath = scene.render.filepath + ".output.exr"
 
     if os.path.basename(filepath).find("merge") != -1:
         # Merge Images