Currently, there is only one testing library, Junit, which you will use for unit testing. But, since your code will interact with other components, even though they may not be the ones under test, you will have to mock them. Junit is still not enough for writing the test cases. Therefore, you will also need to add Hamcrest to help with creating assertion matches and more. Let's go ahead and add the libraries we will need.

Open your module's build file, update the dependencies to match the following code, and sync the project:

dependencies {
  implementation fileTree(dir: 'libs', include: ['*.jar'])
  implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
  implementation 'com.android.support:appcompat-v7:26.1.0'
  implementation 'com.android.support.constraint:constraint-layout:1.0.2'
  testImplementation 'junit:junit:4.12'
  testImplementation "org.mockito:mockito-all:1.10.19"
  testImplementation "org.hamcrest:hamcrest-all:1.3"
  testImplementation "org.powermock:powermock-module-junit4:1.6.2"
  testImplementation "org.powermock:powermock-api-mockito:1.6.2"
  androidTestImplementation 'com.android.support.test:runner:1.0.1'
  androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
}