You don't need the Java plugin, since the Android will take care of what you need mostly, from what I've seen so far.
I managed to get my Robolectric and junit tests running via this man's blog: http://tryge.com/2013/02/28/android-gradle-build/
My build.gradle file looks like this (where my test files are in the {projectdir}/test directory.
...
// Unit tests
sourceSets {
unitTest {
java.srcDir file('test')
resources.srcDir file('test/resources')
}
}
dependencies {
unitTestCompile files("$project.buildDir/classes/debug")
unitTestCompile 'junit:junit:4.11'
unitTestCompile 'org.robolectric:robolectric:2.1.1'
unitTestCompile 'com.google.android:android:4.0.1.2'
}
configurations {
unitTestCompile.extendsFrom runtime
unitTestRuntime.extendsFrom unitTestCompile
}
task unitTest(type:Test, dependsOn: assemble) {
description = "run unit tests"
testClassesDir = project.sourceSets.unitTest.output.classesDir
classpath = project.sourceSets.unitTest.runtimeClasspath
}
build.dependsOn unitTest