Add autoscaling feature flag in release REST tests (#52096)

The REST tests for autoscaling either need to be skipped in a
non-snapshot build, or alternatively, the feature flag registered so
that autoscaling can be enabled. We prefer the latter approach, as it
allows us to also test autoscaling in non-snapshot builds incrementally,
instead of at the end of development as autoscaling prepares for
release. This commit registers the autoscaling feature flag in REST
tests for non-snapshot builds.

x-pack/plugin/autoscaling/qa/rest/build.gradle

@@ -1,3 +1,4 @@
+import org.elasticsearch.gradle.info.BuildParams
 import org.elasticsearch.gradle.test.RestIntegTestTask
 
 apply plugin: 'elasticsearch.testclusters'
@@ -14,6 +15,9 @@ task restTest(type: RestIntegTestTask) {
 
 testClusters.restTest {
   testDistribution = 'DEFAULT'
+  if (BuildParams.isSnapshotBuild() == false) {
+    systemProperty 'es.autoscaling_feature_flag_registered', 'true'
+  }
   setting 'xpack.autoscaling.enabled', 'true'
   setting 'xpack.security.enabled', 'true'
   extraConfigFile 'roles.yml', file('autoscaling-roles.yml')