I have the following Apex class and test class that is only getting 15/21 lines covered. The part that is not covered is in bold below. Any ideas how I can get the setter covered in this Apex?

public class changeControlExtension {
    public Infrastructure_Change_Control__c cc {get;set;}
     
    public changeControlExtension(ApexPages.StandardController stdCtrl) {
        cc = (Infrastructure_Change_Control__c)stdCtrl.getRecord();
    }
     
    //get the multi-select pick list values
    public List<SelectOption> getMSPicklist {
        get {
            List<SelectOption> options = new List<SelectOption>();
            for(Schema.PicklistEntry obj : Infrastructure_Change_Control__c.Department_Approvals__c.getDescribe().getPicklistValues()) {
                options.add(new SelectOption(obj.getValue(), obj.getLabel()));
            }
            return options;
        }  
        set;
    }
     
    //get and set the multi-select pick list as checkboxes
    public String[] MSItems {
        get {
            List<String> selected = new List<String>();
            List<SelectOption> options = this.getMSPicklist;
            for(SelectOption obj : options) {
                if (this.cc.Department_Approvals__c !=null && this.cc.Department_Approvals__c.contains(obj.getValue()))
                    selected.add(obj.getValue());
            }
            return selected;
        }
        set {
            String selectedCheckBox = '';
            for(String s : value) {
                if (selectedCheckBox == '')
                    selectedCheckBox += s;
                else selectedCheckBox += ';' + s;
            }
            cc.Department_Approvals__c = selectedCheckBox;
        }
    }
}

Test Class
@isTest
public class changeControlExtension_Test {

    private static testmethod void changeControl() {
     
     Case cas = new Case(Subject = 'Testcase');
     insert cas;
     
     Infrastructure_Change_Control__c cc = new Infrastructure_Change_Control__c(Name = 'Test Change', Related_Case__c = cas.id);
     insert cc;
             
     ApexPages.StandardController controller = new ApexPages.StandardController(cc);
     changeControlExtension extension = new changeControlExtension(controller);
     
     Test.startTest();
     List<SelectOption> options = extension.getMSPicklist;
     cc.Department_Approvals__c = options.get(0).getValue();
     String[] items = extension.MSItems;
     Test.stopTest();
     
     System.assertNotEquals(options.get(0).getValue(), null);
     System.assertNotEquals(items, null);
        
    }

}