Add pytest-conda-solvers test suite to conda-libmamba-solver

[danyeaw]

What is the idea?

Run pytest-conda-solvers YAML suite with --conda-solver=libmamba from this repo, without replacing the existing libmamba integration or upstream-conda suites (for now).

Why is this needed?

Libmamba already has deep CLI/integration coverage and re-runs conda upstream with CONDA_TEST_SOLVERS=libmamba. The shared YAML suite is a declarative solve-case baseline that matches classic/rattler and is cheaper to interpret when porting or comparing solvers.

What should happen?

• Install pytest-conda-solvers and check out the hub repo so conda-solver-tests is available.
• Add a CI job that runs the shared suite with --conda-solver=libmamba on Python 3.12+ matrix cells only.
• Document the local command (e.g. in docs/dev/workflows.md).
• Keep existing tests/test_*.py and the upstream conda checkout jobs as-is (for now)

Additional Context

• Finish the expanded shared solver YAML suite conda-incubator/pytest-conda-solvers#60
• This repo still CI-tests Python 3.10, make sure we gate the new job so it does not run on unsupported versions.