ADB Install Fails With INSTALL_FAILED_TEST_ONLY

Looks like you need to modify your AndroidManifest.xml
Change android:testOnly="true" to android:testOnly="false" or remove this attribute.

If you want to keep the attribute android:testOnly as true you can use pm install command with -t option, but you may need to push the apk to device first.

$ adb push bin/hello.apk /tmp/
5210 KB/s (825660 bytes in 0.154s)

$ adb shell pm install /tmp/hello.apk 
    pkg: /tmp/hello.apk
Failure [INSTALL_FAILED_TEST_ONLY]

$ adb shell pm install -t /tmp/hello.apk 
    pkg: /tmp/hello.apk
Success

I was able to reproduce the same issue and the above solved it.

If your APK is outside the device (on your desktop), then below command would do it:

$ adb install -t hello.apk

I had a similar problem with Android Studio 3.0.0 Beta 7 and could not publish anymore to the play store.

As stated here: https://developer.android.com/studio/run/index.html

Note: The Run button builds an APK with testOnly="true", which means the APK can only be installed via adb (which Android Studio uses). If you want a debuggable APK that people can install without adb, select your debug variant and click Build > Build APK(s).

Same goes for release build, with Android Studio 3 you need to go to Build > Build APK(s) to have a non testable release apk that you can submit to the store.