Make slow pytest tests opt-in by default

[marko1olo]

Fixes #1327.

This makes tests marked slow opt-in by default for normal pytest runs, while preserving explicit ways to include them:

• --run-slow includes slow tests.
• -m slow and other explicit marker expressions mentioning slow are left to pytest.
• Regular CI jobs can stop spelling -m "not slow"; nightly pytest and incremental coverage still pass --run-slow.

The new option lives in the repository-level conftest.py, so pytest knows about it before collection and before wrapper scripts pass --run-slow.

Local validation:

• python -m pytest src/openfermion/testing/pytest_config_test.py -q -> 4 passed
• python -m pytest src/openfermion/ops/representations/doci_hamiltonian_test.py -q -> 12 passed, 1 deselected
• python -m pytest src/openfermion/ops/representations/doci_hamiltonian_test.py -q --run-slow -> 13 passed
• python -m black --check conftest.py src/openfermion/testing/pytest_config_test.py
• python -m pylint conftest.py src/openfermion/testing/pytest_config_test.py
• parsed .github/workflows/ci.yaml and .github/workflows/nightly-pytest.yaml with PyYAML
• git diff --check origin/main..HEAD

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request configures pytest to skip tests marked as 'slow' by default, introducing a new --run-slow command-line option to include them. It updates the documentation, helper scripts, and configuration files accordingly, and adds unit tests to verify the behavior. One issue was identified in conftest.py where a potential TypeError could occur if the -m option returns None, and a code suggestion was provided to handle this safely.

[gemini-code-assist]

If config.getoption("-m") returns None (which can happen in certain environments or when pytest is run programmatically), calling _SLOW_MARKER_RE.search(markexpr) will raise a TypeError: expected string or bytes-like object. We should ensure markexpr is a truthy string before performing the regex search.

Suggested change

	markexpr = config.getoption("-m", default="")
	if config.getoption("--run-slow") or _SLOW_MARKER_RE.search(markexpr):
	markexpr = config.getoption("-m", default="")
	if config.getoption("--run-slow") or (markexpr and _SLOW_MARKER_RE.search(markexpr)):