Add support for expect-tests

[Alizter]

There are some run time dependencies that are not met when the test runner is run within mutaml-runner.

Here is what I tried:

$ mutaml-runner _build/default/test/expect-tests/dune_engine/.dune_engine_test.inline-tests/inline_test_runner_dune_engine_test.exe
Could not read file mutaml-mut-files.txt - _build/default/mutaml-mut-files.txt: No such file or directory

$ mutaml-runner "dn build @test/expect-tests/dune_engine/runtest --profile release --instrument-with mutaml --always-show-command-line --force"
Could not read file mutaml-mut-files.txt - _build/default/mutaml-mut-files.txt: No such file or directory

This is after

$ dn build @test/expect-tests/dune_engine/runtest --instrument-with mutaml --profile release

(I have to use release since some fatal warnings are being triggered).

(dn is just a bash alias for dune).

It would be great to get this working for expect tests, I've been wanting to try it out on the dune codebase.