[tests] Update TestMultitenantAdminMixin to use default operator group

In this test utility code (which is reused across openwisp modules):

https://github.com/openwisp/openwisp-users/blob/cf5cd64f7d3c3515fcd0fc1f23077620b5a85964/openwisp_users/tests/utils.py#L23-L39

The test expects the permission of the operator to be defined as a class attribute.

This is obsolete, because in more recent versions of OpenWISP we added two default auth.Group instances: operator and administrator.

• update the test method to assign those groups
• remove operator_permission_filters = []
• remove get_operator_permissions

Open a pull request in each openwisp module to remove the operator_permission_filters and ensuring the new code works: https://github.com/search?q=org%3Aopenwisp+operator_permission_filters&type=Code

[niteshsinha17]

I am working on this Issue 😊

[lehone-hp]

Okay, I will assign the appropriate permissions and then open a PR