Add tests for event rule processing

[peteeckel]

Add some tests to validate that the event rule processing in NetBox works properly.

• Only one event per object is triggered in case of multiple save() and M2M field change operations
• The pre- and postchange snapshots reflect the object status before and after the operation

This is mainly a validation that the issues reported in #89, #137, and #277 are fixed and that the NetBox fix is effective for the affected models in NetBox DNS.

Some of the new tests are skipped for all NetBox versions < 4.0.4 (assuming PR 16366 will be included in that release) as they are bound to fail as long as that PR is not merged.