[Ingest Manager] Add tests for the limited concurrency routes

[jfsiii]

From #71552 & #71552 (review)

would like to see test coverage added for limited_concurrency.ts. At the minimum validate that we can get 429 status codes, and also validate that the counter decreases accordingly.

Tests are started in b8288ae

See https://github.com/elastic/kibana/blob/master/x-pack/plugins/security/server/authorization/app_authorization.test.ts for examples

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[jfsiii]

closed by #72338