Test run failed: Instrumentation run failed due to 'java.lang.ClassNotFoundException'

I had upgraded to androidx libraries and started getting this error.

To fix it, in build.gradle I changed the line:

testInstrumentationRunner 'android.support.test.runner.AndroidJUnitRunner'

to

testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

FYI I ran into this and had to add

<uses-library android:name="android.test.runner"/>

within my <application/> tag

In my case, I was trying to run the tests on Jelly Bean emulator. Android instrumentation does not seem to support Jelly Bean. I could run the tests on lollipop.

You may also not able to run android tests through release build type. Ensure that debug build type is active.