Uses-cases of generated dune files

See #3886

The idea for this issue is to gather different use-cases where we currently use generated dune files, with the aim of identifying some common patterns and see if we can provide a better workflow for them.

To get the ball rolling, this is how we use generated dune files at LexiFi:

• generating stanzas for executables (that correspond to tests) listed in an external file (the file itself is static, not generated). The executables are run using a dedicated driver that collects running time, implements a timeout policy, etc so we can't use a simple test stanza for them.
• generating stanzas for compiling C# executables. Some of the files referenced in the stanza (.dll and .cs sources) are listed in external files that are shared in a couple of different places.
• generating copy or copy_files stanzas for files listed in external text files (some of which are generated because they depend on the system type). The list of files are also used in a couple of different places so they need to be shared.
• generating rules for producing "output" files for each executable in a testsuite (the output files are then compared using a specialized program, so cannot use the usual "expect test" workflow). The list of tests and their characteristics are currently specified inside the OCaml script that generates the dune file.

cc @bobot

Start with issue #3886 and review the generated dune-file use cases listed here, including test executables, C# executables, copied files, and test output rules. Done means identifying common patterns and requirements that could support a better workflow; this issue does not name implementation files or tests.