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 from feature-t/3657-fuse-pre in 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test Tools
passed #234863
test_tools

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