A test rule to allow testing coroutines that use the main dispatcher. Without this you'd run into "java.lang.IllegalStateException: Module with the Main dispatcher had failed to initialize. For tests Dispatchers.setMain from kotlinx-coroutines-test module can be used"

build.gradle

......

dependencies {
    def coroutinesVersion = "1.3.0-M1"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$coroutinesVersion"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:$coroutinesVersion"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-test:$coroutinesVersion"


    testImplementation 'org.hamcrest:hamcrest-library:2.1'
    testImplementation group: 'org.mockito', name: 'mockito-core', version: '2.25.0'

    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.2.0'
    androidTestImplementation 'androidx.test:rules:1.2.0'

    testImplementation group: 'org.mockito', name: 'mockito-core', version: '2.25.0'
}

CoroutineTestRule.kt

package com.aniketkadam.sharevideoshortcut
import org.junit.rules.TestWatcher

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.test.TestCoroutineDispatcher
import kotlinx.coroutines.test.resetMain
import kotlinx.coroutines.test.setMain
import org.junit.runner.Description

@ExperimentalCoroutinesApi
class CoroutineTestRule(private val dispatcher = TestCoroutineDispatcher()) : TestWatcher() {

    override fun starting(description: Description?) {
        super.starting(description)
        Dispatchers.setMain(dispatcher)
    }

    override fun finished(description: Description?) {
        super.finished(description)
        Dispatchers.resetMain()
    }
    
}

ExampleUseage.kt

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.test.runBlockingTest
import org.junit.Before
import org.junit.Rule
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4
import org.mockito.Mock
import org.mockito.Mockito.*
import org.mockito.MockitoAnnotations
import org.mockito.Mockito.`when` as wh

@RunWith(JUnit4::class)
class ExampleTest {

    @ExperimentalCoroutinesApi
    @get:Rule
    val coroutineTestRule = CoroutineTestRule()

    @ExperimentalCoroutinesApi
    @Test
    fun exampleTest() = runBlockingTest { 
        val result = suspendingFunction()
	assert(result)
    }
}

@AniketSK thank you very much! just adding CoroutineTestRule class and following lines to my test class helped me:

@ExperimentalCoroutinesApi
@get:Rule
val coroutineTestRule = CoroutineTestRule()

class CoroutineTestRule(private val dispatcher = TestCoroutineDispatcher()) : TestWatcher() {

to

class CoroutineTestRule(private val dispatcher: CoroutineDispatcher = TestCoroutineDispatcher()) : TestWatcher() {