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

The Python test suite now fails wit exit code 4 if no tests ran. It
should help detecting typos in test names and test methods.

* Add "EXITCODE_" constants to Lib/test/libregrtest/main.py.
* Fix a typo: "NO TEST RUN" becomes "NO TESTS RAN"