[Meta] Migrate SLO/Alerting/Synthetics api integration tests to deployment agnostic solution

[mgiota]

🍒 Summary

After #183113 is merged, we have a way to write deployment agnostic tests for SLO and o11y alerting features.

Test	Type	ESS	Serverless	Notes
Custom threshold rule type	Alerting	✅	✅	Duplicate tests Serverless: x-pack/test_serverless/api_integration/test_suites/observability/custom_threshold_rule ESS: x-pack/test/alerting_api_integration/observability/custom_threshold_rule
Metric threshold rule type	Alerting	✅	❌	ESS: x-pack/test/alerting_api_integration/observability/metric_threshold_rule.ts
ES query rule type	Alerting	❌	✅	ESS: x-pack/test_serverless/api_integration/test_suites/observability/es_query_rule
SLO tests	SLO	✅	✅	Different implementation in both environments. Serverless: x-pack/test_serverless/api_integration/test_suites/observability/slos ESS: x-pack/test/api_integration/apis/slos

The above table is a compilation of our current API integration tests for the slo and alerting features. Some tests are duplicated in both environments, others are implemented either in serverless or ESS environments and there are also some tests (SLO) where the implementation is not the same in both environments and should be unified.

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)