Make Python test suite fail if no tests ran

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When the Python test suite is run with a typo in a test filename or a test method, it displays "NO TEST RUN" but the exit code is a success (exit code 0) and so the mistake can be easily missed. I propose to make the test suite fail with a non-zero exit code to highlight that something unusual happened.

• PR: gh-98903: Test suite fails with exit code 4 if no tests ran #98904

[vstinner]

Non-zero exit code should also help automation of running the Python test suite, with git bisect for example.

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Fixed by c76db37