[8.11] [TSVB] Try to stabilize the test (elastic#169254) (elastic#169407

)

# Backport

This will backport the following commits from `main` to `8.11`:
- [[TSVB] Try to stabilize the test
(elastic#169254)](elastic#169254)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Stratoula
Kalafateli","email":"efstratia.kalafateli@elastic.co"},"sourceCommit":{"committedDate":"2023-10-19T14:24:57Z","message":"[TSVB]
Try to stabilize the test (elastic#169254)\n\n## Summary\r\n\r\nHopefully
stabilizes this flakiness\r\n\r\nCloses
https://github.com/elastic/kibana/issues/167728\r\n\r\nRun 100
times:\r\nhttps://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/3593","sha":"1fcdd3428ce6188e8456f0f4d14abe95487401b6","branchLabelMapping":{"^v8.12.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Feature:TSVB","Team:Visualizations","failed-test","release_note:skip","backport:prev-minor","v8.11.0","v8.12.0"],"number":169254,"url":"https://github.com/elastic/kibana/pull/169254","mergeCommit":{"message":"[TSVB]
Try to stabilize the test (elastic#169254)\n\n## Summary\r\n\r\nHopefully
stabilizes this flakiness\r\n\r\nCloses
https://github.com/elastic/kibana/issues/167728\r\n\r\nRun 100
times:\r\nhttps://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/3593","sha":"1fcdd3428ce6188e8456f0f4d14abe95487401b6"}},"sourceBranch":"main","suggestedTargetBranches":["8.11"],"targetPullRequestStates":[{"branch":"8.11","label":"v8.11.0","labelRegex":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"},{"branch":"main","label":"v8.12.0","labelRegex":"^v8.12.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/169254","number":169254,"mergeCommit":{"message":"[TSVB]
Try to stabilize the test (elastic#169254)\n\n## Summary\r\n\r\nHopefully
stabilizes this flakiness\r\n\r\nCloses
https://github.com/elastic/kibana/issues/167728\r\n\r\nRun 100
times:\r\nhttps://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/3593","sha":"1fcdd3428ce6188e8456f0f4d14abe95487401b6"}}]}]
BACKPORT-->

Co-authored-by: Stratoula Kalafateli <efstratia.kalafateli@elastic.co>

test/functional/page_objects/visual_builder_page.ts

@@ -667,6 +667,7 @@ export class VisualBuilderPageObject extends FtrService {
     const fieldEl = await this.getFieldForAggregation(aggNth);
 
     await this.comboBox.setElement(fieldEl, field);
+    await this.header.waitUntilLoadingHasFinished();
   }
 
   public async setFieldForAggregateBy(field: string): Promise<void> {