bpo-37224: Improve test__xxsubinterpreters.DestroyTests (GH-18058)

Adds an additional assertion check based on a race condition for `test__xxsubinterpreters.DestroyTests.test_still_running` discovered in the bpo issue.


https://bugs.python.org/issue37224

Author: aeros

Lib/test/test__xxsubinterpreters.py

@@ -759,7 +759,11 @@ def test_still_running(self):
         main, = interpreters.list_all()
         interp = interpreters.create()
         with _running(interp):
-            with self.assertRaises(RuntimeError):
+            self.assertTrue(interpreters.is_running(interp),
+                            msg=f"Interp {interp} should be running before destruction.")
+
+            with self.assertRaises(RuntimeError,
+                                   msg=f"Should not be able to destroy interp {interp} while it's still running."):
                 interpreters.destroy(interp)
             self.assertTrue(interpreters.is_running(interp))