[ML] Increase Test Coverage 8.1.0

[walterra]

Follow up to #110949 and #117930.

Meta issue to summarize improvements to test coverage for 8.1.0.

Jest unit tests

API integration tests

• POST /api/ml/datafeeds/:datafeedId/_update (including spaces test) (JG)
  [ML] Adding update datafeed API test #124540
• POST /api/ml/anomaly_detectors/:jobId/_forecast (PH)
  [ML] Adds anomaly detectors forecast API test #125214
• GET /api/ml/data_frame/analytics/:analyticsId/messages (MA)
  [ML] Data Frame Analytics audit messages api test #125325
• POST /api/ml/results/category_definition (JG)
  [ML] Adding category definition and category examples api tests #124710
• POST /api/ml/results/category_examples (JG)
  [ML] Adding category definition and category examples api tests #124710

Functional tests

• Transforms: Retention Policy (WR)
  [ML] Transforms: Fix editing retention policy without default value. #123450
  [ML] Transforms: Fix retention policy reset #124698
• Transforms: Reset Action (WR)
  [ML] Transforms: Adds reset action to transforms management. #123735
• Transforms: Terms agg (WR)
  [ML] Transforms: Support for terms agg in pivot configurations. #123634
• Transforms: Ingest Pipelines (WR)
  [ML] Transforms: Support to set destination ingest pipeline. #123911
• Data visualizer to Lens shows chart (QN)
  [ML] Assert Lens show chart after navigation from Index data visualizer #124579
• Saved searches in DFA creation + cloning (MA)
  [ML] Data Frame Analytics saved search creation functional tests #125040
• Overview page for new design (DA)
  [ML] Functional tests for the Overview page and side nav #124793
• DFA models list row expansion (DA)
  [ML] Fix Trained Model stats and pipelines tab  #125382

Accessibility tests

part of #88496

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

Closing issue as all the tests marked for 8.1 have now been added.