meta tests: refactor run_pytest

[ikonst]

Factor run_pytest out of mypy/test/meta/test_*.py.

Also, prefixing the nested pytest output so it's harder to mix it up with parent pytest's:

>> .../mypy/venv/bin/python -m pytest -n 0 -s mypy/test/testcheck.py::TypeCheckSuite::check-meta-909158d4-c9c7-41e6-be50-cf101aaa2b56.test
NESTED PYTEST STDOUT: ============================= test session starts ==============================
                    : platform darwin -- Python 3.11.3, pytest-7.3.1, pluggy-1.0.0
                    : rootdir: .../mypy