I have an Android application using the new standard gradle layout:
src/main/java src/main/resources src/instrumentTest/java src/instrumentTest/resources
You can use the assets folder of the build type your are testing. If you are testing the debug type (which is the default one):
src/debug/res/assets
And then you access those files with
InputStream raw = context.getAssets().open("filename.ext");
String myJson = new Scanner(raw).useDelimiter("\\A").next();
The trick of using the debug build type prevent your testing resources to be packed in the final apk.