Try import pybind11_cross_module_tests from test_namespace_visibility.py

Author: rwgk

tests/test_namespace_visibility.py

@@ -1,9 +1,11 @@
 import namespace_visibility_1
 import namespace_visibility_2
+import pybind11_cross_module_tests as cm
 import pytest
 
 
 def test_namespace_visibility():
+    del cm  # Unused.
     modules = (
         namespace_visibility_1,
         namespace_visibility_2,