fuse.py: pre-processing with --pre

- Define a function that could be used for post-processing
- Use defs.py to determine where the pre-process are stored
- Should tests

See #3647
1 job for <span class="ref-name">feature-t/3657-fuse-pre</span> in 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test Tools
passed #234863
test_tools

00:00:07