Include Non-OCaml/Reason Files in Dune Build?

I'm new to OCaml and Dune, and I'm trying to set up a basic project with test cases. I want my test module to be able to read in text files from a subdirectory and process the input of those files. However, although I'm reading the Dune documentation, I'm having trouble finding the right stanzas to tell Dune to include a directory with no .ml files in it.

When I build my Dune project, it doesn't include the directory where I put my test files.

|-project  |-lib  |-test    |-test_files (* Not included in _build *)      test_input1.txt      test_input2.txt    tests.ml

It seems like a lot of stanzas, like include_subdirs, tell Dune to look for modules, but it appears that Dune will ignore non-module files. Is that what's going on? How do I work around this?