[Build] Add test admin when starting gradle run with trial license and

also add more documentation around gradle run task.

Author: martijnvg

TESTING.asciidoc

@@ -25,6 +25,37 @@ run it using Gradle:
 ./gradlew run
 -------------------------------------
 
+==== Distribution
+
+By default a node is started with the zip distribution.
+In order to start with a different distribution use the `-Drun.distribution` argument.
+
+To for example start the open source distribution:
+
+-------------------------------------
+./gradlew run -Drun.distribution=oss-zip
+-------------------------------------
+
+==== License type
+
+By default a node is started with the `basic` license type.
+In order to start with a different license type use the `-Drun.distribution` argument.
+
+In order to start a node with a trail license execute the following command:
+
+-------------------------------------
+./gradlew run -Dlicense_type=trial
+-------------------------------------
+
+This enables security and other paid features and adds a superuser with the username: `elastic-admin` and
+password: `elastic-password`.
+
+==== Other useful arguments
+
+In order to start a node with a different max heap space add: `-Dtests.heap.size=4G`
+In order to disable annotations add: `-Dtests.asserts=false`
+In order to set an Elasticsearch setting, provide a setting with the following prefix: `-Dtests.es.`
+
 === Test case filtering.
 
 - `tests.class` is a class-filtering shell-like glob pattern,

distribution/build.gradle

@@ -316,6 +316,8 @@ task run(type: RunTask) {
       setting 'xpack.graph.enabled', 'true'
       setting 'xpack.watcher.enabled', 'true'
       setting 'xpack.license.self_generated.type', 'trial'
+      setupCommand 'setupTestAdmin',
+        'bin/elasticsearch-users', 'useradd', 'elastic-admin', '-p', 'elastic-password', '-r', 'superuser'
     } else if (licenseType != 'basic') {
       throw new IllegalArgumentException("Unsupported self-generated license type: [" + licenseType + "[basic] or [trial].")
     }