[Feature] centralized Python tests and default to single-process pytest to avoid CUDA OOM

[viiccwen]

What

• Move QDP Python tests from qdp/qdp-python/tests/ to testing/qdp_python/ so all Python tests live under the top-level testing/ tree and are run by make test_python / root pytest.
• Change the default Python test run to single-process (no -n auto) to avoid CUDA out-of-memory and flakiness when multiple workers use the GPU; document that users with sufficient compute can optionally use -n auto or -n N to speed up.

Why

1. Test location: QDP Python tests live under qdp/qdp-python/tests/, while the rest of the Python suite is under testing/ (e.g. testing/qumat, testing/qdp). Root pytest uses testpaths = ["testing"], so the qdp-python tests were not discovered by the default make test_python run unless invoked from the right place or with an extra path.
2. Parallelism: Running the full suite with -n auto can cause CUDA OOM or instability when several workers use the GPU. A single-process default is safer; users who have enough GPU memory and want faster runs should be able to opt in to parallelism via documentation.