Android Studio Test Folder not appearing for Unit Test Artifact

The androidTest appeared on my Android Studio when the Test Artifact was switched to Android Instrumentation Test. However, when the Test Artifact was switched back to Unit Test. The entire androidTest folder disappear. This project was a migration from Eclipse. I have multiple flavor for my project.

Gradle.build

sourceSets {
    main {
       // manifest, java, renderscript, aidl, assets, res detail etc etc
    }
    flavour1 {
       // manifest, java, renderscript, aidl, assets, res detail etc etc
    }
    flavour2 {
       // manifest, java, renderscript, aidl, assets, res detail etc etc
    }
    flavour3 {
       // manifest, java, renderscript, aidl, assets, res detail etc etc
    }
    flavour4 {
       // manifest, java, renderscript, aidl, assets, res detail etc etc
    }
    androidTest.setRoot('tdd/test')
    androidTest {
         java.srcDirs = ['tdd/test/java']
    }
}

I don't understand why it disappear for Unit Test but appear for Android Instrumentation Test.