Ensure the set of gems tested at unit level matches packages

This commit adds the installDefaultGems task to the unit test tasks. This
ensures that the gem env tested at the unit level matches the deduplicated one
at the integration/acceptance level. Takes over elastic#18330

Author: donoghuc

build.gradle

@@ -418,6 +418,7 @@ project(":logstash-core") {
     tasks.getByPath(":logstash-core:" + tsk).configure {
       dependsOn copyPluginTestAlias
       dependsOn installDevelopmentGems
+      dependsOn installDefaultGems
     }
   }
 }
@@ -1001,6 +1002,7 @@ if (System.getenv('OSS') != 'true') {
     ["rubyTests", "rubyIntegrationTests", "test"].each { tsk ->
       tasks.getByPath(":logstash-xpack:" + tsk).configure {
         dependsOn installDevelopmentGems
+        dependsOn installDefaultGems
       }
     }
   }