[coq] [coq-lang-1.0] Ensure test suite is comprehensive.

Before (coq lang 1.0) we need to ensure that we are testing all possible configurations of Coq Dune projects.

It seems there are quite a few, let's study the variables:

• composition: single theory, multiple same-scope ones, multiple theories in multiple workspaces
• Coq: not present, in workspace, installed
• stdlib present: no, in workspace, installed
• user-contrib: no, installed
• coqpath: yes / no
• stdlib flag: yes / no
• theories visibility: public and private, can depend in 3 different ways x 2 different configurations (same / different scope)
• native: enabled / disabled in config, manually overriden.
• plugins: no / yes single module / yes depending on other libs
• plugins: living in coqpath without findlib registration / with findlib registration
• transitive deps: both in worksspace and in user-contrib
• Coq versions: maybe 8.13 / 8.16 / 8.17 ?
• library names: different kind of overlappings

That's a lot of combos to test, but can be reduced with a bit of care.

Another point to consider is how to have a naming scheme that help us identify what part of the test matrix the test is covering.

Start by surveying the existing Coq Dune test suite and its current organization. Map the configuration dimensions listed in the issue, identify which combinations are already covered, and propose a naming scheme that shows the covered matrix. Done means the relevant configurations are deliberately covered without unnecessary combinations.