Fix project configuration: avoid adding test jar as library dependency

For some reason gradle processes "testJar" task differently and collects
artifacts for it during configuration phase. This helps Idea project
configurator understand that there's no need to attach test jar as separate
binary library dependency.

Having compiled module in dependencies results IDEA finds both compiled and
source version of classes. This may cause bad navigation and reindexing.

buildSrc/src/main/kotlin/artifacts.kt

@@ -36,10 +36,12 @@ fun Project.classesDirsArtifact(): FileCollection {
     return classesDirs
 }
 
+private const val MAGIC_DO_NOT_CHANGE_TEST_JAR_TASK_NAME = "testJar"
+
 fun Project.testsJar(body: Jar.() -> Unit = {}): Jar {
     val testsJarCfg = configurations.getOrCreate("tests-jar").extendsFrom(configurations["testCompile"])
 
-    return task<Jar>("testsJar") {
+    return task<Jar>(MAGIC_DO_NOT_CHANGE_TEST_JAR_TASK_NAME) {
         dependsOn("testClasses")
         pluginManager.withPlugin("java") {
             from(project.the<JavaPluginConvention>().sourceSets.getByName("test").output)