Use pytest's pytester plugin in the tests

There's lots of tests in `tests/test_pytest_mpl.py` which write test code to a temp file, run it, and check the result. Pytest's builtin pytester plugin is designed for doing this and has lots of nice features for checking the outcome.

https://docs.pytest.org/en/7.1.x/how-to/writing_plugins.html#testing-plugins

Start with tests/test_pytest_mpl.py and read pytest's pytester plugin documentation at the linked guide. Replace the temporary-file execution patterns with pytester-based tests while preserving the existing outcome checks; done means the test suite passes with equivalent coverage.

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