[Discover][Embeddable] Fix search highlighting (elastic#197607)

- Closes elastic#197195

## Summary

This PR fixes the search highlights for saved search panels on
Dashboard.


### 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

src/plugins/discover/public/embeddable/utils/update_search_source.ts

@@ -52,6 +52,7 @@ export const updateSearchSource = (
 ) => {
   const { sortDir } = defaults;
   searchSource.setField('size', sampleSize);
+  searchSource.setField('highlightAll', true);
   searchSource.setField(
     'sort',
     getSortForSearchSource({

test/functional/apps/discover/embeddable/_saved_search_embeddable.ts

@@ -19,6 +19,7 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
   const esArchiver = getService('esArchiver');
   const kibanaServer = getService('kibanaServer');
   const testSubjects = getService('testSubjects');
+  const find = getService('find');
   const { common, dashboard, header, discover } = getPageObjects([
     'common',
     'dashboard',
@@ -143,5 +144,19 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
       await addSearchEmbeddableToDashboard();
       await testSubjects.existOrFail('unifiedDataTableToolbar');
     });
+
+    it('should display search highlights', async () => {
+      await addSearchEmbeddableToDashboard();
+      await queryBar.setQuery('Mozilla');
+      await queryBar.submitQuery();
+      await header.waitUntilLoadingHasFinished();
+      await dashboard.waitForRenderComplete();
+      const marks = await find.allByCssSelector('.unifiedDataTable__cellValue mark');
+      const highlights = await Promise.all(
+        marks.map(async (highlight) => await highlight.getVisibleText())
+      );
+      expect(highlights.length).to.be.greaterThan(0);
+      expect(highlights.every((text) => text === 'Mozilla')).to.be(true);
+    });
   });
 }