add instructions for customization of ./gradlew run (#88484)

TESTING.asciidoc

@@ -108,6 +108,40 @@ password: `elastic-password`.
 - In order to set an Elasticsearch setting, provide a setting with the following prefix: `-Dtests.es.`
 - In order to pass a JVM setting, e.g. to disable assertions: `-Dtests.jvm.argline="-da"`
 
+==== Customizing the test cluster for ./gradlew run
+
+You may need to customize the cluster configuration for the ./gradlew run task.
+You can simply find the task in the source code and configure it there.
+(The task is currently defined in build-tools-internal/src/main/groovy/elasticsearch.run.gradle)
+However, this requires modifying a source controlled file and subject to accidental commits.
+Alternatively, you can use a Gradle init script to inject custom build logic with the -I flag to configure this task locally.
+
+For example:
+
+To enable HTTPS for use with ./gradlew run, an extraConfigFile is needed to be added to the cluster configuration.
+Create a file (for example ~/custom-run.gradle) with the following contents:
+-------------------------------------
+rootProject {
+  if(project.name == 'elasticsearch') {
+    afterEvaluate {
+        testClusters.matching { it.name == "runTask"}.configureEach {
+          extraConfigFile 'http.p12', file("<path/to/file>/http.p12")
+        }
+    }
+  }
+}
+-------------------------------------
+Now tell Gradle to use this init script:
+-------------------------------------
+./gradlew run -I ~/custom-run.gradle \
+-Dtests.es.xpack.security.http.ssl.enabled=true \
+-Dtests.es.xpack.security.http.ssl.keystore.path=http.p12
+-------------------------------------
+
+Now the http.p12 file will be placed in the config directory of the running cluster and available for use.
+Assuming you have the http.ssl.keystore setup correctly, you can now use HTTPS with ./gradlew run without the risk
+of accidentally committing your local configurations.
+
 === Test case filtering.
 
 You can run a single test, provided that you specify the Gradle project. See the documentation on