[not verified] Add newsletter categories mock function for testing

In the subscriptions module, a mock function was added in the test/edit.js file. This was done to simulate the behavior of the actual 'useNewsletterCategories' function in the unit tests. This will facilitate the testing process by controlling its response and help in improving the code coverage.

projects/plugins/jetpack/extensions/blocks/subscriptions/test/edit.js

@@ -73,10 +73,20 @@ jest.mock( '@wordpress/element', () => ( {
 
 jest.mock( '@wordpress/notices', () => {}, { virtual: true } );
 
+jest.mock( '../hooks/use-newsletter-categories', () => ( {
+	useNewsletterCategories: () => {
+		return {
+			data: [],
+			enabled: false,
+			error: false,
+			loading: false,
+		};
+	},
+} ) );
+
 describe( 'SubscriptionEdit', () => {
 	test( 'adds correct classes to container', async () => {
 		const { container } = render( <SubscriptionEdit { ...defaultProps } /> );
-
 		// eslint-disable-next-line testing-library/no-container, testing-library/no-node-access
 		expect( container.querySelector( `.${ defaultProps.className }` ) ).toBeInTheDocument();
 	} );