Add missing DECREF to PyErr_WriteUnraisable(). That function reports

exceptions that can't be raised any further, because (for instance) they
occur in __del__ methods. The coroutine tests in test_generators was
triggering this leak. Remove the leakers' testcase, and add a simpler
testcase that explicitly tests this leak to test_generators.

test_generators now no longer leaks at all, on my machine. This fix may also
solve other leaks, but my full refleakhunting run is still busy, so who
knows?

Python/errors.c

@@ -604,6 +604,7 @@ PyErr_WriteUnraisable(PyObject *obj)
 				PyFile_WriteString(": ", f);
 				PyFile_WriteObject(v, f, 0);
 			}
+			Py_DECREF(moduleName);
 		}
 		PyFile_WriteString(" in ", f);
 		PyFile_WriteObject(obj, f, 0);