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.