[tests] Add selenium test `test_multiple_organization_templates`

[Dhanus3133]

The PR #529 removed the broken test test_device_templates_m2m_queryset, which had already been removed from openwisp/openwisp-controller#1010.

To maintain test coverage for this functionality, we should add and run the Selenium-based test test_multiple_organization_templates, which serves as an equivalent.