[ML] Transform: Fix <DefinePivotSummary /> jest test. (elastic#195998)

## Summary

Fixes elastic#195992 (failing test).

Updates the mock for the preview endpoint to make the test pass again.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#_add_your_labels)

(cherry picked from commit c4d6403)

x-pack/plugins/transform/public/app/__mocks__/app_dependencies.tsx

@@ -67,7 +67,14 @@ dataStart.search.search = jest.fn(({ params }: IKibanaSearchRequest) => {
 }) as ISearchGeneric;
 
 // Replace mock to support tests for `use_index_data`.
-coreSetup.http.post = jest.fn().mockResolvedValue([]);
+coreSetup.http.post = jest.fn().mockImplementation((endpoint) => {
+  if (endpoint === '/internal/transform/transforms/_preview') {
+    return Promise.resolve({
+      generated_dest_index: { mappings: { properties: {} } },
+      preview: [],
+    });
+  }
+});
 
 const appDependencies: AppDependencies = {
   analytics: coreStart.analytics,

x-pack/plugins/transform/public/app/sections/create_transform/components/step_define/step_define_summary.test.tsx

@@ -20,9 +20,7 @@ import { StepDefineSummary } from './step_define_summary';
 
 jest.mock('../../../../app_dependencies');
 
-// Failing: https://github.com/elastic/kibana/issues/195992
-describe.skip('Transform: <DefinePivotSummary />', () => {
-  // Using the async/await wait()/done() pattern to avoid act() errors.
+describe('Transform: <DefinePivotSummary />', () => {
   test('Minimal initialization', async () => {
     // Arrange
     const queryClient = new QueryClient();