Add tests for filtesets

### Plugin Version

v0.8.1

### Proposed functionality

Add tests for filtersets. The following filtersets are in branching (`BranchEventFilterSet`, `BranchFilterSet`, `ChangeDiffFilterSet`) but have no test cases.  NetBox has test cases for all filtersets.


### Use case

Currently the filtersets have no test, so regressions will not be found.

### External dependencies

N/A