[ML] Increase Test Coverage 7.16

[walterra]

Follow up to #106113.

Meta issue to summarize improvements to test coverage for 7.16.

Jest unit tests

API integration tests

part of #63700

• GET /api/ml/datafeeds (including spaces test) [ML] Adding datafeed api tests #116133 (JG)
• GET /api/ml/datafeeds/:datafeedId (including spaces test) [ML] Adding datafeed api tests #116133 (JG)
• GET /api/ml/datafeeds/_stats (including spaces test) [ML] Adding datafeed api tests #116133 (JG)
• GET /api/ml/datafeeds/:datafeedId/_stats (including spaces test) [ML] Adding datafeed api tests #116133 (JG)
• POST /api/ml/validate/datafeed_preview [ML] Adding ability to change data view in advanced job wizard #115191 (JG)
• Transform health rule (transform not started) [Transform] Transforms health alerting rule type #112277 (DA)

Functional tests

• Forecasts [ML] Anomaly Detection: Functional tests for anomaly detection forecasts. #116140 (WR)
• DFA models row content and delete action [ML] Fix deletion of models that are not used by pipelines #114107 (PH)
• Dashboard with ML embeddables migration (e.g. load 7.15 JSON into 7.16) [ML] Add tests for anomaly embeddables migrations #116520 (QN)

Accessibility tests

part of #88496

APM Correlations API integration tests

These are tests updated/integrated as part of the refactor away from search strategies to regular endpoints in #115145

• 'GET /internal/apm/correlations/field_candidates'
• 'GET /internal/apm/correlations/field_value_pairs'
• 'POST /internal/apm/latency/overall_distribution'
• 'POST /internal/apm/correlations/p_values'
• 'POST /internal/apm/correlations/significant_correlations'
• Overall correlations analysis workflow test
• Overall failed transactions correlations analysis workflow test

[elasticmachine]

Pinging @elastic/ml-ui (:ml)