Other referenced package cannot be found in test - unit-testing

Question

In my project (if you want detailed information, you can find it on Github) I've got two modules:
app (the default module generated when you start out with a new Android project)
dice (new module, which is a dice rolling domain model package)
I've started out fleshing out my MVP classes in the app module, and I want to reuse the PredefinedRandomIntegerGenerator class (on Github) from the dice module (residing the test folder of that module) in my unit tests in the app module.
When I started writing my test code I encountered no problems, until I started running my
C:\Projects\Dice-Statistics\app\src\test\java\com\xilconic\dicestatistics\viewmodels\SixSidedDieRollingViewModelTest.java:5: error: package com.xilconic.gaming.dice.test.mocks does not exist
import com.xilconic.gaming.dice.test.mocks.PredefinedRandomIntegerGenerator;
^
C:\Projects\Dice-Statistics\app\src\test\java\com\xilconic\dicestatistics\viewmodels\SixSidedDieRollingViewModelTest.java:21: error: cannot find symbol
RandomIntegerGenerator rng = new PredefinedRandomIntegerGenerator(results);
^
symbol: class PredefinedRandomIntegerGenerator
location: class SixSidedDieRollingViewModelTest
I have no clue why during the build the com.xilconic.gaming.dice.test.mocks package can't be found. Am I missing something in my Gradle script for the app module? That looks as follows:
apply plugin: 'com.android.application'
android {
compileSdkVersion 23
buildToolsVersion "23.0.2"
defaultConfig {
applicationId "com.xilconic.dicestatistics"
minSdkVersion 8
targetSdkVersion 23
versionCode 1
versionName "1.0"
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
}
}
}
dependencies {
compile fileTree(include: ['*.jar'], dir: 'libs')
testCompile 'junit:junit:4.12'
compile 'com.android.support:appcompat-v7:23.1.1'
compile 'com.android.support:design:23.1.1'
compile project(':dice')
}
Any help would be very welcome!