Add option to view unassigned tests in a test run

[s-leavens]

Is your feature request related to a problem? Please describe.
When viewing a test run, I can select an assignee and see everything assigned to them, but I can't do that for unassigned.

Describe the solution you'd like
I'd like unassigned to be a selectable option so I can see everything in a test run that hasn't been assigned yet.

[TetianaKhomenko]

@littlefrontender please check if we can do this

[littlefrontender]

@TetianaKhomenko This should be done on the backend