[data_views/_edit_field] update test (#174062)

## Summary

This test was failing on MKI with retry timeout. 

https://buildkite.com/elastic/appex-qa-serverless-kibana-ftr-tests/builds/748

![download](https://github.com/elastic/kibana/assets/10977896/e841ee44-00b1-4364-9af7-33b1ff2682d9)

I update it to search for particular child element we are validating.
Not sure if it solves the flakiness, but now if it fails we know there
is something wrong with `data-test-subj='fieldPreviewItem'` element.

x-pack/test_serverless/functional/test_suites/common/management/data_views/_edit_field.ts

@@ -34,26 +34,20 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
       it('should show preview for fields in _source', async function () {
         await PageObjects.settings.filterField('extension');
         await testSubjects.click('editFieldFormat');
-        await testSubjects.find('value');
-        let previewText = '';
-        await retry.waitForWithTimeout('get preview value', 1000, async () => {
-          previewText = await testSubjects.getVisibleText('value');
-          return previewText !== 'Value not set';
+        await retry.tryForTime(5000, async () => {
+          const previewText = await testSubjects.getVisibleText('fieldPreviewItem > value');
+          expect(previewText).to.be('css');
         });
-        expect(previewText).to.be('css');
         await PageObjects.settings.closeIndexPatternFieldEditor();
       });
 
       it('should show preview for fields not in _source', async function () {
         await PageObjects.settings.filterField('extension.raw');
         await testSubjects.click('editFieldFormat');
-        await testSubjects.find('value');
-        let previewText = '';
-        await retry.waitForWithTimeout('get preview value', 1000, async () => {
-          previewText = await testSubjects.getVisibleText('value');
-          return previewText !== 'Value not set';
+        await retry.tryForTime(5000, async () => {
+          const previewText = await testSubjects.getVisibleText('fieldPreviewItem > value');
+          expect(previewText).to.be('css');
         });
-        expect(previewText).to.be('css');
         await PageObjects.settings.closeIndexPatternFieldEditor();
       });
     });