Improve unit test isolation. - #28

Better isolates the unit test environment and location to avoid future conflicts with the integration test and due to dependency divergence.

Summary of changes in this pull request

• test-unit and module are now 'married' during solving and lockfile generation, while keeping test-unit dependencies out of the module conda lockfiles.
• unit tests have their own placement to avoid conftest.py conflicts between regular integration tests and unit tests.
• Should also fix the Windows tests failure (cause was the deletion of an open file, which is not allowed on that OS).

Reviewer checklist

• There are no pip dependencies in the module's environment files (workflow/envs/).
• All rules use pathvars (e.g., <results>) in their inputs and outputs.
• The integration test-suite is successful, including:
  • pre-commit.ci tests pass.
  • tests pass for all relevant OS configurations (linux, osx, windows).
• Module documentation is up-to-date, including:
  • INTERFACE.yaml mentions all relevant pathvars and wildcards.
  • README.md describes how to use the module and has the necessary citations.