Document how to use a Roboelectric test dependency for Android unit tests that use pbandk

[garyp]

If pbandk's JSON functionality (e.g. Message.encodeToJsonString()) is used from within an Android unit test, the unit test fails to run because the JVM used for running unit tests does not have an android.util.Base64 implementation. android.* dependencies are only available to tests in the androidTest source set, which run on real or emulated devices rather than on the local JVM.

Robolectric provides a robust way to run unit tests that include Android dependencies on the JVM by simulating many Android APIs without running an actual device; but it requires using the Robolectric test runner and modifying tests to initialize the simulated environment appropriately. However, there is a lighter-weight way of using Robolectric if UI-related Android APIs aren't needed (Context, Activity, etc.). In that case, all that's needed is a dependency on the platform-specific Robolectric runtime library:

testRuntimeOnly("org.robolectric:android-all:11-robolectric-6757853")

If the above dependency is added to the Android project that uses pbandk, then pbandk's JSON-related methods will run successfully from unit tests.