Cycles tests: Don't create fail file on idiff warning

tests/python/cycles_render_tests.py

@@ -70,6 +70,7 @@ def verify_output(filepath):
             os.remove(failed_image)
         return True
     except subprocess.CalledProcessError as e:
+        if e.returncode != 1:
             shutil.copy(TEMP_FILE, failed_image)
         if VERBOSE:
             print(e.output.decode("utf-8"))