How can OCaml dune test program find test input files?

── test
    ├── dune
    ├── test_program.ml
    └── foo_files
        ├── foo_a
        └── foo_b

How can test_program find the files under folder foo_files when run using dune test?

I can see that when the program is run, the current working directory is _build/default/test and the foo_files folder is not copied there.

dune 3.15.3

Basically, dune will only copy to _build/default the things that are flagged as dependencies of the targets it is supposed to build. In your case, the dune file of test might look like:

; tells dune that some rules might depend on foo_files' content
; see https://dune.readthedocs.io/en/stable/reference/dune/data_only_dirs.html
(data_only_dirs foo_files)

(test
  (name test_program)
  (deps (source_tree foo_files))
  ; other fields if needed
)