Add tests for filters service

[Arif-Khalid]

Is your feature request related to a problem? Please describe. Filters service is a service introduced uniquely to watcher, it therefore does not have any test cases.

Describe the solution you'd like Comprehensive test cases for the filters service.

• [ ] Test initial behaviour
• [ ] Test update filters
• [ ] Test update preset view
• [ ] Test clear filters
• [ ] Test unique filter updating functions
• [ ] Test unique preset view updating functions
• [ ] Test URL initialization
• [ ] Test label sanitization
• [ ] Test milestone sanitization
• [ ] Test get currently active milestones

[Arif-Khalid]

As I created the filters service and want to get some experience in writing tests, I will work on this.