Error "No tests found" when running Android instrumentation tests

check build.gradle

replace testInstrumentationRunner "androidx.test.runner.AndroidJUnit4"

android {
    defaultConfig {
        testInstrumentationRunner 'androidx.test.runner.AndroidJUnitRunner'
    }
}

Look for app/src/androidTest/AndroidManifest.xml

change android:name to androidx.multidex.MultiDexApplication

<application
        android:allowBackup="true"
        android:label="Components Tests App"
        android:supportsRtl="true"
        android:name="android.support.multidex.MultiDexApplication">

To

<application
            android:allowBackup="true"
            android:label="Components Tests App"
            android:supportsRtl="true"
            android:name="androidx.multidex.MultiDexApplication">

More suggestions: No tests found when running instrumented tests with AndroidX

If you are targeting SDK Version 28, you'll be using the re-packaged androidx classes from the support library, so AndroidJUnitRunner is configured like so...

android {
    defaultConfig {
        testInstrumentationRunner 'androidx.test.runner.AndroidJUnitRunner'
    }
}

You have forgotten to set AndroidJUnitRunner as the default test instrumentation runner.

https://developer.android.com/topic/libraries/testing-support-library/index.html

To set AndroidJUnitRunner as the default test instrumentation runner in your Gradle project, specify this dependency in your build.gradle file:

android {
    defaultConfig {
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
}

---

Update

with androidx things have changed a little, check https://developer.android.com/training/testing/set-up-project