先说下故事的背景。此时的as已经升级到了3.1.2（>3.0版本），需要给测试一个测试app进行测试！如此so easy的事，直接run了运行了一下，就搞定了
谁知，测试那边说app装不了，也挺搞人的！一想不对啊，以前在2.3.1版本都是可以的啊！没啥问题啊
各种查找资料后，有篇技术博客说了：
https://stackoverflow.com/questions/25274296/adb-install-fails-with-install-failed-test-only
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.意思大概说：在as3.0以后，直接运行的（run）的app，其伴有的属性是testonly=“true”，意味着该app只能在as上运行开发！如果想弄个测试app，需要点击 Build –>Build apk即可
所以，以后给测试app就跑一下Build apk即可了