build.gradle

dependencies {
    testImplementation 'junit:junit:4.12'
    testImplementation "androidx.arch.core:core-testing:2.0.0-rc01"
    testImplementation "com.nhaarman.mockitokotlin2:mockito-kotlin:2.0.0-alpha03@jar"
}

ExampleUnitTest.kt

import androidx.arch.core.executor.testing.InstantTaskExecutorRule
import androidx.lifecycle.Observer
import com.nhaarman.mockitokotlin2.mock
import com.nhaarman.mockitokotlin2.verify
import org.junit.Rule
import org.junit.Test

class ExampleUnitTest {
    @Rule
    @JvmField
    val rule = InstantTaskExecutorRule()

    @Test
    fun testLiveData() {
        // Given
        val meuViewModel = MeuViewModel()
        val observer = mock<Observer<Int>>()
        meuViewModel.getValor().observeForever(observer)
        // When
        meuViewModel.atualizarValor(10)
        // Then
        verify(observer).onChanged(10)
    }
}

MeuVideModel.kt

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class MeuViewModel : ViewModel() {
    private val valor = MutableLiveData<Int>()

    fun getValor(): LiveData<Int> = valor

    fun atualizarValor(v: Int) {
        valor.value = v
    }
}