OZG-7473 replace string with const

Based on CR comment.

alfa-client/libs/admin/aggregation-mapping/src/lib/aggregation-mapping-form-container/aggregation-mapping-form/aggregation-mapping-field-list-form/aggregation-mapping-field-form/aggregation-mapping-field-form.component.spec.ts

 import { ADMIN_FORMSERVICE } from '@admin-client/shared';
 import { EMPTY_STRING } from '@alfa-client/tech-shared';
-import { existsAsHtmlElement, mock, Mock, mockGetValue, triggerEvent } from '@alfa-client/test-utils';
+import { existsAsHtmlElement, mock, Mock, MockEvent, mockGetValue, triggerEvent } from '@alfa-client/test-utils';
 import { ComponentFixture, TestBed } from '@angular/core/testing';
 import { FormBuilder, FormControl, FormGroup } from '@angular/forms';
 import { expect } from '@jest/globals';
@@ -78,7 +78,7 @@ describe('AggregationMappingFieldFormComponent', () => {
       });
 
       it('should remove mapping on click', () => {
-        triggerEvent({ fixture, elementSelector: removeMappingButtonTestId, name: 'clickEmitter', data: fieldIndex });
+        triggerEvent({ fixture, elementSelector: removeMappingButtonTestId, name: MockEvent.CLICK, data: fieldIndex });
 
         expect(formService.removeMapping).toHaveBeenCalledWith(fieldIndex);
       });