[APM] Enable skipped e2e tests

[gbamparop]

Some flaky e2e tests that were causing builds to fail have been skipped as part of https://github.com/elastic/kibana/pull/136118.

Skipped tests in the following files will need to be enabled:

• [ ] read_only_user/dependencies.spec.ts
• [ ] read_only_user/errors/error_details.spec.ts
• [ ] read_only_user/service_inventory/service_inventory.spec.ts
• [ ] read_only_user/service_overview/header_filters.spec.ts
• [ ] read_only_user/service_overview/instances_table.spec.ts
• [ ] read_only_user/service_overview/service_overview.spec.ts [@cauemarcondes ]
• [x] power_user/feature_flag/comparison.ts [@cauemarcondes ][https://github.com/elastic/kibana/pull/137209]
• [x] power_user/feature_flag/infrastructure.spec.ts [@cauemarcondes ] [https://github.com/elastic/kibana/pull/137780]

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[MiriamAparicio]

Closed here https://github.com/elastic/kibana/pull/139912