Android Studio 3.0 sets android:testOnly="true" automatically on APKs that are ran from the IDE.

However, for some devices (such as OPPO R11), the test-only APKs can never be installed …

If you face the same problem, try add this line to your ‘/gradle.properties’:

# gradle.properties
android.injected.testOnly=false

Then the boring android:testOnly attribute disappears. ( ͡° ͜ʖ ͡°)✧

---

BTW, if you do not have an ‘OPPO R11’ (congratulations), and just want to install such a test-only app with adb, try:

adb install -t <test_only_apk_filename>.apk

Otherwise, you will get a failure message:

Failed to install app-debug.apk: Failure [INSTALL_FAILED_TEST_ONLY: installPackageLI]