[APM] Write tests for the Custom Link API

[cauemarcondes]

Add new tests on the feature controls test:
https://github.com/elastic/kibana/blob/b83f81458c11cbb890023e662920f3679c615ab7/x-pack/test/api_integration/apis/apm/feature_controls.ts
Create API test following: https://github.com/elastic/kibana/blob/master/x-pack/legacy/plugins/apm/readme.md#api-integration-tests

[elasticmachine]

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