Android instrumented test no tests found

Please add the following into your build.gradle and put your test classes into androidTest folder

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

I recently tried the accepted answer, but it gave me a process crash error when trying to run. If you get this you should use:

testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

Source: Instrumentation run failed due to 'Process crashed.'