Unable to Cover Test Case with both debounce and takeUntil,filter Angular
filter(item=>!item) means that it will only passtrought FALSY values (null,undefined,false,""). What you want is the other way around - pass only truthy values
filter(item=>item) or filter(item=>!!item)
After OP editing question:
Swap detectChanges and tick
it should be
fixture.detectChanges(); //kicks in ngOnInit
tick(2000); // simulates passage of time