How do I prevent a flavor specific unit test from being compiled in another flavor?

I'm trying to do this, but with a single dimension so it's simpler: How to specify unit test folder in two-dimension flavor

app module build.gradle specifies 2 flavors, free and premium:

    apply plugin: 'com.android.application'
    apply plugin: 'kotlin-android'
    apply plugin: 'kotlin-android-extensions'

    android {
        compileSdkVersion 28
        defaultConfig {
        applicationId "com.example.myapplication"
        minSdkVersion 15
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner 
    "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android- 
     optimize.txt'), 'proguard-rules.pro'
        }
    }
    flavorDimensions 'someFlavor'
    productFlavors {
        premium {
            applicationIdSuffix '.premium'
        }

        free
    }
    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
        debug.java.srcDirs += 'src/debug/kotlin'
        release.java.srcDirs += 'src/release/kotlin'

        free {
            java.srcDirs += 'src/free/kotlin'
            test.java.srcDirs += 'src/test/kotlin'
        }

        premium {
            java.srcDirs += 'src/premium/kotlin'
            test.java.srcDirs += ['src/test/kotlin', 
        'src/testPremium/kotlin']
        }
    }
}

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation"org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
implementation 'com.android.support:appcompat-v7:28.0.0'
implementation 'com.android.support.constraint:constraint-layout:1.1.3'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.2'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
implementation 'com.android.support:design:28.0.0'
}

Android Studio's Project view shows:

app
  src
    free
      kotlin (empty folder for now)
    main
      java
        com.example.myapplication
          MainActivity.java
    premium
      kotlin
        com.example.myapplication
          PremiumFile.kt
    test
      kotlin
        com.example.myapplication
          MainTest.kt
    testPremium
      kotlin
        com.example.myapplication
          PremiumTest.kt

PremiumFile.kt

class PremiumFile {
  companion obnject {
    const val PREMIUM_NUM = 10
  }
}

MainTest.kt

class MainTest {
    @Test
    fun basicMainTest() {
      assertEquals(1, 1)
    }
}

PremiumTest.kt

class PremiumTest {
    @Test
    fun premiumTest() {
        assertEquals(PremiumFile.Premium_NUM, 1)
    }
}

As you can see, I lack any unit tests specific to the free flavor. I expected that running ./gradlew compileFreeReleaseUnitTestKotlin would compile the shared unit test inside src/test, but it also compiles the Premium unit tests inside src/testPremium. How can I make sure that PremiumTest.kt is only compiled for Premium tasks such as ./gradlew compilePremiumReleaseUnitTestKotlin?

from How do I prevent a flavor specific unit test from being compiled in another flavor?