getting Failure [INSTALL_FAILED_TEST_ONLY: installPackageLI] in Android studio 3.0

You can also use command like this :

adb install -r -t myapp.apk

it works for me:

PS C:\Users\languoguang> adb -P 12345 install -r D:\GreeneTrans\HelloWorld-signed.apk
adb: failed to install D:\GreeneTrans\HelloWorld-signed.apk: Failure [INSTALL_FAILED_TEST_ONLY: installPackageLI]
PS C:\Users\languoguang> adb -P 12345 install -t D:\GreeneTrans\HelloWorld-signed.apk
Success
PS C:\Users\languoguang> adb -P 12345 install -r -t D:\GreeneTrans\HelloWorld-signed.apk
Success
PS C:\Users\languoguang>