[MKI] Fix flaky search source alert test (elastic#205481)

## Summary

Use settings
[method](https://github.com/elastic/kibana/blob/c8d46ee94982cf38163786f0890bafd43ec07247/test/functional/page_objects/settings_page.ts#L648)
instead of naive typing.
Naive only because the asterisk at the end of the text box is better
handled in the settings method.

x-pack/test_serverless/functional/test_suites/common/discover_ml_uptime/discover/search_source_alert.ts

@@ -553,9 +553,9 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
 
       // change title
       await testSubjects.click('editIndexPatternButton');
-      await testSubjects.setValue('createIndexPatternTitleInput', 'search-s', {
-        clearWithKeyboard: true,
-        typeCharByChar: true,
+      await PageObjects.header.waitUntilLoadingHasFinished();
+      await retry.try(async () => {
+        await PageObjects.settings.setIndexPatternField('search-s*');
       });
       await testSubjects.click('saveIndexPatternButton');
       await testSubjects.click('confirmModalConfirmButton');