Test whether searchers work without control plane

[rdettai]

Is your feature request related to a problem? Please describe.
We recently made some changes to enable searchers to work without control plane (#5360). A few things should be validated though to ensure this works as expected:

• concerns regarding the correctness of the change Disable control plane check for searcher. #5360 (comment)
• the searchers seem to be configured with a metastore client that is proxied through the CP:

  quickwit/quickwit/quickwit-serve/src/lib.rs

  Lines 604 to 612 in c9f34df

  	let (search_job_placer, search_service) = setup_searcher(
  	&node_config,
  	cluster.change_stream(),
  	metastore_through_control_plane.clone(),
  	storage_resolver.clone(),
  	searcher_context,
  	)
  	.await
  	.context("failed to start searcher service")?;

Describe the solution you'd like
Write an integration test to validate this.

[rdettai]

cc @trinity-1686a