A guide to Activity, Service, and Content Provider Testing by JUnit
Android Develop Team has well introduction on Activity Test by JUnit. Here is the link leads you to explore the testing skill.
For Service Testing by JUnit, take a look WEB sit - http://developer.android.com/tools/testing/service_testing.html.
For Content provider testing by JUnit, take a look WEB sit - http://developer.android.com/tools/testing/contentprovider_testing.html.
The limitation for this method to do test is that you need to have the source code of Activity you want to test. And you need to compile test case with source code of Activity.
One of strength of using JUnit to test Activity is you can control the particular UI component such as a button, or a textarea. Another good standpoint is that you can use JUnit to manage your test case and test result. There's another testing tool to test Android Framework called CTS. The CTS uses JUnit for its testing framework as well. With respect to CTS, take a look following WEB Site for more information. http://source.android.com/compatibility/cts-development.html