Fix flaky test from #172781 (#176972)

## Summary

This PR fixes the flaky test skipped in #172781.

Flaky test runner x95:
https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/5187.

Resolves #172781.

### Checklist

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [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] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
- [ ] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [ ] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

test/examples/unified_field_list_examples/existing_fields.ts

@@ -73,7 +73,7 @@ export default ({ getService, getPageObjects }: FtrProviderContext) => {
       await PageObjects.timePicker.setAbsoluteRange(TEST_START_TIME, TEST_END_TIME);
       await PageObjects.header.waitUntilLoadingHasFinished();
       await PageObjects.unifiedFieldList.waitUntilSidebarHasLoaded();
-      await PageObjects.unifiedFieldList.toggleSidebarSection('meta');
+      await PageObjects.unifiedFieldList.openSidebarSection('meta');
     });
 
     after(async () => {

test/functional/page_objects/unified_field_list.ts

@@ -88,6 +88,22 @@ export class UnifiedFieldListPageObject extends FtrService {
     );
   }
 
+  public async openSidebarSection(sectionName: SidebarSectionName) {
+    const openedSectionSelector = `${this.getSidebarSectionSelector(
+      sectionName,
+      true
+    )}.euiAccordion-isOpen`;
+
+    if (await this.find.existsByCssSelector(openedSectionSelector)) {
+      return;
+    }
+
+    await this.retry.waitFor(`${sectionName} fields section to open`, async () => {
+      await this.toggleSidebarSection(sectionName);
+      return await this.find.existsByCssSelector(openedSectionSelector);
+    });
+  }
+
   public async waitUntilFieldPopoverIsOpen() {
     await this.retry.waitFor('popover is open', async () => {
       return Boolean(await this.find.byCssSelector('[data-popover-open="true"]'));

x-pack/test_serverless/functional/test_suites/common/examples/unified_field_list_examples/existing_fields.ts

@@ -78,7 +78,7 @@ export default ({ getService, getPageObjects }: FtrProviderContext) => {
       await PageObjects.timePicker.setAbsoluteRange(TEST_START_TIME, TEST_END_TIME);
       await PageObjects.header.waitUntilLoadingHasFinished();
       await PageObjects.unifiedFieldList.waitUntilSidebarHasLoaded();
-      await PageObjects.unifiedFieldList.toggleSidebarSection('meta');
+      await PageObjects.unifiedFieldList.openSidebarSection('meta');
     });
 
     after(async () => {
@@ -92,8 +92,7 @@ export default ({ getService, getPageObjects }: FtrProviderContext) => {
       await kibanaServer.savedObjects.cleanStandardList();
     });
 
-    // FLAKY: https://github.com/elastic/kibana/issues/172781
-    describe.skip('existence', () => {
+    describe('existence', () => {
       it('should find which fields exist in the sample documents', async () => {
         const sidebarFields = await PageObjects.unifiedFieldList.getAllFieldNames();
         expect(sidebarFields.sort()).to.eql([...metaFields, ...fieldsWithData].sort());