why android studio says 'Test events were not received'?

I'm trying to unit test in my android application, and this is the simple test tutorial what i'm doing.

import static org.junit.Assert.*;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.robolectric.RobolectricTestRunner;

@RunWith(RobolectricTestRunner.class)
public class ServerListManagerTest extends AndroidTestCase{

   @Test
   public void testTrueIsTrue() throws Exception {
    assertEquals(true, true);
    }
}

The directory is like this, src\main\androidTest\java\some packages\ServerListManagerTest.java

I tried changing directory of this, and also build configuration. but android studio still doesn't recognize my unit test though build was successful.

This is my build.gradle in app,

apply plugin: 'com.android.application'

android {
   compileSdkVersion 21
   buildToolsVersion "21.1.2"

   defaultConfig {
       applicationId "com.kaist.se.pmpapp"
       minSdkVersion 16
       targetSdkVersion 21
       versionCode 1
       versionName "1.0"
   }

buildTypes {
       release {
          minifyEnabled false
          proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
         }
      }
sourceSets { main { java.srcDirs = ['src/main/java', 'src/androidTest/java'] } } }



dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:22.1.1'
    androidTestCompile 'org.robolectric:robolectric:2.4'
    androidTestCompile 'junit:junit:4.12'
    androidTestCompile group: 'junit', name: 'junit', version: '4.12'
  }

What's wrong in my code????