Build: Setup standalone tests to compile in intellij

This adds the standalone tests so they will compile (and thus can be
modified with import completion) within IntelliJ. It also explicitly
sets up buildSrc as a module.

Note that this does *not* mean eg evil-tests can be run from intellij.
These are special tests that require special settings (eg disabling
security manager). They need to be run from the command line.

closes #15075

build.gradle

@@ -177,6 +177,12 @@ tasks.idea.doLast {
 if (System.getProperty('idea.active') != null && ideaMarker.exists() == false) {
   throw new GradleException('You must run gradle idea from the root of elasticsearch before importing into IntelliJ')
 }
+// add buildSrc itself as a groovy project
+task buildSrcIdea(type: GradleBuild) {
+  buildFile = 'buildSrc/build.gradle'
+  tasks = ['cleanIdea', 'ideaModule']
+}
+tasks.idea.dependsOn(buildSrcIdea)
 
 
 // eclipse configuration

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/StandaloneTestBasePlugin.groovy

@@ -39,9 +39,6 @@ class StandaloneTestBasePlugin implements Plugin<Project> {
         BuildPlugin.globalBuildInfo(project)
         BuildPlugin.configureRepositories(project)
 
-        // remove some unnecessary tasks for a qa test
-        project.tasks.removeAll { it.name in ['assemble', 'buildDependents'] }
-
         // only setup tests to build
         project.sourceSets {
             test
@@ -56,6 +53,12 @@ class StandaloneTestBasePlugin implements Plugin<Project> {
                 plusConfigurations = [project.configurations.testRuntime]
             }
         }
+        project.idea {
+            module {
+                testSourceDirs += project.sourceSets.test.java.srcDirs
+                scopes['TEST'] = [plus: [project.configurations.testRuntime]]
+            }
+        }
         PrecommitTasks.configure(project)
     }
 }