My project layout is roughly as follows:

/
├── src/
│   ├── dune
│   ├── libname.ml
│   └── libname.mli
└── test/
    ├── dune
    └── testname.ml

The test/dune file contains:

(test
 (name testname)
 (modes byte exe)
 (libraries libname)
)

So it runs the test both in bytecode and exe mode.

The library built in src depends on a dynamically-loaded native library that is output to _build/default/src. When I run dune runtest test --force --always-show-commandline, I get the following output:

(cd _build/default/test && ./testname.exe)
Ran
File "test/dune", line 2, characters 7-18:
2 |  (name testname)
           ^^^^^^^^
(cd _build/default/test && ./testname.bc)
Fatal error: cannot load shared library dlltlapm_lib_stubs
Reason: dlltlapm_lib_stubs.so: cannot open shared object file: No such file or directory

So the exe run finds the the shared library without problem, but the bytecode run does not. I can fix this by modifying OCAML_LD_LIBRARY_PATH as follows, since the --always-show-command-line output tells us that dune is cd‘ing into _build/default/test:

CAML_LD_LIBRARY_PATH=../src:$CAML_LD_LIBRARY_PATH

but using relative paths in this way is very fragile and won’t work if I start putting tests in subdirectories of the test dir. How can I reliably tell dune runtest where to look for shared libraries?