[BUG] opensearch-extra plugin testing does not work

[finnegancarroll]

Describe the bug

This section of documentation in TESTING.md seems stale maybe? I can't find any mention of this special 'opensearch-extra' directory in the code and have had no luck using this feature to build a plugin alongside opensearch.

Related component

Build

To Reproduce

git clone https://github.com/opensearch-project/OpenSearch.git
mkdir OpenSearch/opensearch-extra && git clone https://github.com/opensearch-project/asynchronous-search.git OpenSearch/opensearch-extra/asynchronous-search
cd OpenSearch && ./gradlew run

Expected behavior

Based on the documentation I'm expecting OpenSearch to build and include the plugin cloned into 'opensearch-extra'.
I don't see my plugin with either of the below.

./gradlew projects
curl -X GET "http://localhost:9200/_cat/plugins"

Additional Details

No response

[dblock]

That seems incorrect. Last time I was testing a plugin where I needed to make modifications in both core and the plugin I did #15528 (comment). Maybe you can help update the documentation one way or another?