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 (https://github.com/quickwit-oss/quickwit/pull/5360). A few things should be validated though to ensure this works as expected:

• concerns regarding the correctness of the change https://github.com/quickwit-oss/quickwit/pull/5360#discussion_r1735870060
• the searchers seem to be configured with a metastore client that is proxied through the CP: https://github.com/quickwit-oss/quickwit/blob/c9f34df6507f6ac9ab5d66d667b186e8e1ede97a/quickwit/quickwit-serve/src/lib.rs#L604-L612

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

[rdettai]

cc @trinity-1686a