Fix #4166: Make test_create_already look in the process's output instead of only stdout

... (supposedly) some recent dependency change made it so
the (human-readable) error output lands in stderr instead
of stdout (as it should be).

Fixes [ticket:4166](https://tahoe-lafs.org/trac/tahoe-lafs/ticket/4166)

Co-authored-by: Chris Wood <chris@leastauthority.com>
Co-authored-by: Jeff Jacobson <jeffjacobsonhimself@gmail.com>

src/allmydata/test/cli/test_grid_manager.py

@@ -90,7 +90,7 @@ class GridManagerCommandLine(TestCase):
             self.assertEqual(1, result.exit_code)
             self.assertIn(
                 "Can't create",
-                result.stdout,
+                result.output,
             )
 
     def test_create_stdout(self):