Adopt pytest for the existing test suite (discovery + fixtures)

[bernalde]

Goal

Make the existing test suite discoverable and runnable by pytest without changing test logic or reformatting source code. This is the foundation for restoring CI (part of #3).

Current state

• Two test modules exist, both unittest.TestCase-based and run only via python path/to/file.py:
  • tests/integration/reactor_tests.py — PlugFlowReactorTests (test_mole_conc, test_temperature), data-driven from tests/integration/data/pfr_test_expected_conc_*.csv (51 files).
  • tests/Flowsheet/flowsheet_tests.py — TestFlowsheets with 4 multi-unit scenarios, data in tests/Flowsheet/data/.
• No conftest.py, no pytest.ini, no tests/__init__.py.
• Test data is loaded from relative paths, so tests fail when invoked from another working directory.
• unittest.TestCase discovers fine under pytest, so no rewrite is required.

Tasks

• Add tests/__init__.py (and tests/integration/__init__.py, tests/Flowsheet/__init__.py as needed).
• Add tests/conftest.py with fixtures:
  • data_path → resolves Path(__file__).parent / ... so tests are CWD-independent.
  • assimulo_available → returns whether assimulo imports; used to skip integration tests gracefully.
• Add pytest.ini (or [tool.pytest.ini_options] in pyproject.toml) with:
  • testpaths = tests, discovery for *_tests.py / test_*.py.
  • markers: slow, integration, unit, assimulo.
  • a sane default timeout and warning filters.
• Update the two test modules to resolve data files via the data_path fixture / Path(__file__) instead of relative strings (minimal, surgical change only).
• Mark assimulo-dependent tests with @pytest.mark.assimulo and auto-skip when unavailable.

Acceptance criteria

• pytest tests/ -v collects and runs both modules from any working directory.
• Assimulo-dependent tests are skipped (not errored) when assimulo is absent.
• No source files outside tests/ are modified; no black/formatting changes.

Reference

Reusable scaffolding existed in reverted PR #106 (pytest.ini, tests/conftest.py at commit 599ffb3) — adapt it, but keep this PR scoped to test discovery only.

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Upstream PR

Wave 1 — strong first candidate. Test-only, touches no source files; ideal low-risk PR to open directly against CryPTSys/PharmaPy master. See the upstream strategy in #3.

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Reverted PR provenance

This issue is part of the selective re-introduction of useful infrastructure from the reverted upstream work:

• Implement CI/CD pipeline and enhance testing framework for PharmaPy #1 and Refactor codebase and enhance CI for quality checks #2: original fork-side CI/testing work.
• Implement CI/CD pipeline and enhance testing framework for PharmaPy CryPTSys/PharmaPy#106: upstream PR that bundled CI, packaging, tests, docs, and broad formatting.
• Revert "Implement CI/CD pipeline and enhance testing framework for PharmaPy" CryPTSys/PharmaPy#107: upstream revert of that bundled PR.

Implementation should stay small and reviewable, avoid repo-wide formatting, and preserve traceability through one focused PR per issue.

[bernalde]

Validation before merging #90:

• python -m pytest --collect-only -q -rs -> 7 tests collected.
• python -m pytest tests/ -m "unit" -rs -> 1 passed, 6 deselected.
• python -m pytest tests/ -m "not assimulo" -rs -> 1 passed, 6 deselected.
• python -m pytest -> 1 passed, 6 skipped because Assimulo is not installed in the active interpreter.
• Out-of-repo invocation from /tmp: python -m pytest /tmp/pharmapy-pr90-address/tests/ -m "not assimulo" -rs -> 1 passed, 6 deselected.

The current PR head is 561aeebc77cdf9ed09cdd3f7dc472bac1776bfaf. No GitHub Actions checks are configured for this branch yet, so there is no remote CI result to report.