PermissionSetTestExample.cls

        PermissionSet permissionSet = new PermissionSet();
        permissionSet.Name='ApexTest';
        permissionSet.label='Apex Test';
        insert permissionSet;
        ObjectPermissions objectPermission = new ObjectPermissions();
        objectPermission.ParentId = permissionSet.Id;
		objectPermission.SObjectType = Book_Order__c.SObjectType.getDescribe().getName();
        objectPermission.PermissionsEdit = true;
        objectPermission.PermissionsRead = true;
        objectPermission.PermissionsDelete = true;        
        objectPermission.PermissionsCreate = true;        
        insert objectPermission;
        ObjectPermissions objectPermission2 = new ObjectPermissions();
        objectPermission2.ParentId = permissionSet.Id;
		objectPermission2.SObjectType = Order_Line_Item__c.SObjectType.getDescribe().getName();
        objectPermission2.PermissionsEdit = true;
        objectPermission2.PermissionsRead = true;
        objectPermission2.PermissionsDelete = true;        
        objectPermission2.PermissionsCreate = true;                
        insert objectPermission2;
        FieldPermissions fieldPermission = new FieldPermissions();
        fieldPermission.ParentId = permissionSet.Id;
        fieldPermission.SObjectType = Order_Line_Item__c.SObjectType.getDescribe().getName(); 
        fieldPermission.Field = Order_Line_Item__c.SObjectType.getDescribe().getName() + '.' + Order_Line_Item__c.Shipping_Status__c.getDescribe().getName();
        fieldPermission.PermissionsEdit = allowUpdateShippingStatus ? true : false;        
        fieldPermission.PermissionsRead = true;
        insert fieldPermission;                 
        User newUser = new User(
			Alias = 'test1'+System.today().dayOfYear(), 
            Email=permissionSet.Id+'@testorg.com', 
            EmailEncodingKey='UTF-8', 
            LastName='Testing', 
            LanguageLocaleKey='en_US', 
            LocaleSidKey='en_US', 
            ProfileId = [select Id from Profile where Name = 'Standard User'].Id, 
            TimeZoneSidKey='America/Los_Angeles', 
            UserName=permissionSet.Id+'@testorg.com');
		insert newUser;
        insert new PermissionSetAssignment(AssigneeId=newUser.Id, PermissionSetId=permissionSet.Id);