tests: Get tests working with Python 3.11 (except with pytest)

We use a common idiom ( https://stackoverflow.com/a/39606065
https://gist.github.com/hynekcer/1b0a260ef72dae05fe9611904d7b9675 )
for getting the results of our unittest.TestCases in the tearDown
method. This method accesses a private property, and that private
property was removed in Python 3.11

The StackOverflow answer has been updated with a new approach for
Python 3.11, which also uses a private property. This fixes things
for CTest (and the test target when building), but it still doesn't
work when using pytest's unittest support.

Ping #18740

test/subprocesstest.py

@@ -165,7 +165,14 @@ class SubprocessTestCase(unittest.TestCase):
         # It remains None when running in debug mode (`pytest --pdb`).
         # The property is available since Python 3.4 until at least Python 3.7.
         if self._outcome:
-            for test_case, exc_info in self._outcome.errors:
+            if hasattr(self._outcome, 'errors'):
+                # Python 3.4 - 3.10
+                result = self.defaultTestResult()
+                self._feedErrorsToResult(result, self._outcome.errors)
+            else:
+                # Python 3.11+
+                result = self._outcome.result
+            for test_case, exc_info in (result.errors + result.failures):
                 if exc_info:
                     return True
         # No errors occurred or running in debug mode.