Hello, I am trying to get code coverage for a trigger, but am having trouble writing unit tests. The purpose of the trigger is to throw an error when a user tries to close a Case with outstanding Tasks associated with the record. Here is the trigger

public class OpenTaskHandler {
    
    public static void getCaseList(List<Case> triggerNew, Map<id, Case> newMap){
    	Set<Case> newlyClosedCaseIds = new Set<Case>();
        
    	for (Case caseId : newlyClosedCaseIds)
    	{
        	if (caseId.IsClosed)
        		{
                	newlyClosedCaseIds.add(caseId);
            	}
    	}
    
    	for (AggregateResult aggResult : [ 
        	SELECT Count(Id), WhatId From Task WHERE WhatId In :newlyClosedCaseIds AND isClosed = false
        	Group by WhatId
        	having Count(Id) > 0
    	]) {
        	Case openCase = (Case) aggResult.get('WhatId');
        	Case errorCase = newMap.get(openCase.Id);
        
        	errorCase.addError('Cannot close case since there are non closed tasks: ' + errorCase.Id);
    	}
}

Here is one of my tests:

@isTest public class CaseTaskTriggers {
    @isTest public static void TestOpenTasks() {
        
       Case testCase = new Case();
        testCase.Status='New';
        insert testCase;
        
       Task tsk = new Task();
        tsk.Subject = 'New hire has been added';
        tsk.WhatId = [select Id from Case Limit 1].Id;
        tsk.OwnerId = UserInfo.getUserId();
        tsk.Status = 'New';
        insert tsk; 
        
        Test.startTest();
        
        try{
            testCase.Status = 'closed';
            update testCase;
        }
        Catch(Exception ee){}
        
        system.assertNotEquals(tsk.Status = 'Closed', tsk.Status = 'New', 'Cannot close case since there are non closed tasks');
        
        test.stopTest();
    }

I am getting red code coverage on the if (caseId.IsClosed) {newlyClosedCaseIds.add(caseId), the Case openCase = (Case) aggResult.get('whatid') and the errorCase.addError. How do I account for these on a unit test? I am fairly new to unit testing and would love any tips/feedback. Thanks !