[work-in-progress] Test pytest vs python

Suggestion from pytest-dev/pytest#11138 (comment)

With 'pytest' (launching the whole pylint test suite):
FAILED tests/test_precedence.py::test_package - AssertionError: E: 21: Module 'package.AudioTime' has no 'DECIMAL' member<function Equals.<locals>.<lambda> at 0x76c566741750>

With 'pytest tests/test_precedence.py':
tests/test_precedence.py .                                                                                                                                                                           [100%]

============================================================================================ 1 passed in 1.04s =============================================================================================

With 'python tests/test_precedence.py':
Checked ['package.__init__'] successfully
Checked ['precedence_test'] successfully
Checked ['import_package_subpackage_module'] successfully
Checked ['pylint.checkers.__init__'] successfully
Checked ['/home/pierre/pylint/tests/regrtest_data/classdoc_usage.py'] successfully
Checked ['/home/pierre/pylint/tests/regrtest_data/module_global.py'] successfully
Checked ['/home/pierre/pylint/tests/regrtest_data/decimal_inference.py'] successfully
Checked ['/home/pierre/pylint/tests/regrtest_data/absimp/string.py'] successfully
Checked ['/home/pierre/pylint/tests/regrtest_data/bad_package'] successfully

tests/test_precedence.py

@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+import os
+import sys
+from collections.abc import Callable
+from os.path import abspath, dirname, join
+
+from pylint import checkers
+from pylint.lint.pylinter import PyLinter
+from pylint.testutils import GenericTestReporter
+
+REGR_DATA = join(dirname(abspath(__file__)), "regrtest_data")
+sys.path.insert(1, REGR_DATA)
+
+
+def Equals(expected: str) -> Callable[[str], bool]:
+    return lambda got: got == expected
+
+
+def test_package() -> None:
+    for file_names, check in [
+        (["package.__init__"], Equals("")),
+        (["precedence_test"], Equals("")),
+        (["import_package_subpackage_module"], Equals("")),
+        (["pylint.checkers.__init__"], lambda x: "__path__" not in x),
+        ([join(REGR_DATA, "classdoc_usage.py")], Equals("")),
+        ([join(REGR_DATA, "module_global.py")], Equals("")),
+        ([join(REGR_DATA, "decimal_inference.py")], Equals("")),
+        ([join(REGR_DATA, "absimp", "string.py")], Equals("")),
+        ([join(REGR_DATA, "bad_package")], lambda x: "Unused import missing" in x),
+    ]:
+        finalize_linter = PyLinter()
+        finalize_linter.set_reporter(GenericTestReporter())
+        checkers.initialize(finalize_linter)
+        os.environ.pop("PYLINTRC", None)
+        finalize_linter.reporter.finalize()
+        finalize_linter.check(file_names)
+        got = finalize_linter.reporter.finalize().strip()
+        assert check(got), str(got) + str(check)
+        print(f"Checked {file_names} successfully")
+
+
+if __name__ == "__main__":
+    test_package()