Add tests for searching in a new library

[EmileTrotignon]

For now, there are two kinds of search tests : search in made up libraries, and search in the base library. It would be beneficial to test the search on more libraries.

Here is a list of potential libraries :

• brr
• cmdliner
• tyxml
• alcotest

You are welcome to pick something outside of this list.

Files to edit are :

• dune-project
• test/dune

You will also need to create a new .t file in test/cram. It can be based on test/cram/base_cli.t .