So you decided get some Unit tests running in JUnit except how the hell does one get started in libGDX in Android using Gradle. I pulled my hair out so hopefully you don't have to; here are a few steps I did which may help you move forward.

Create the directory and a test file

Create a directory where our tests will sit in

mkdir -p core/test/java/org/package/name
touch core/test/java/org/package/name/WorkingTest.java

The file core/src/test/java/org/package/name/WorkingTest.java has this content for our example

package org.package.name;
import org.junit.Ignore;
import org.junit.Test;

import static org.junit.Assert.assertTrue;

public class WorkingTest 
{
    @Test
    public void thisAlwaysPasses() 
    {
        assertTrue(true);
    }

    @Test
    @Ignore
    public void thisIsIgnored() 
    {
    }
}

Setting up Gradle

First we need to tell that junit is a dependency which needs to be compiled. Below shows a snippet of projects build.gradle in the root folder:

...
project(":core") {
    apply plugin: "java"

    sourceSets.test.java.srcDirs = ["test/"]

    dependencies {

        compile "com.badlogicgames.gdx:gdx:$gdxVersion"                                    
        compile "com.badlogicgames.gdx:gdx-box2d:$gdxVersion"                              
        compile "com.badlogicgames.gdx:gdx-freetype:$gdxVersion"                           
        compile fileTree(dir: 'libs', include: '*.jar')                                    

        testCompile "junit:junit:4.11"                                                     
        testCompile "com.badlogicgames.gdx:gdx-backend-headless:$gdxVersion"               
        testCompile "com.badlogicgames.gdx:gdx:$gdxVersion"                                
        testCompile "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-desktop"       
        testCompile "com.badlogicgames.gdx:gdx-box2d:$gdxVersion"                          
        testCompile "com.badlogicgames.gdx:gdx-box2d-platform:$gdxVersion:natives-desktop" 
    }
}
...

The compile "junit:junit:4.11+" is the line we added, note you can change this to junit:junit:+ to be more generic but the example above is what I have running

Ignoring some files which are conflicting when building

If you try to build for android you might get some APK LICENSE.txt errors such as:

Duplicate files at the same path inside the APK: LICENSE.txt

To fix this we need to tell the android build process to exclude any files that have the name LICENSE.txt. To do this you can edit your android/build.gradle file and add the packagingOption option in the android section. For example it should look like this:

android {
    buildToolsVersion "19.0.3"
    compileSdkVersion 19
    sourceSets {
        main {
            manifest.srcFile 'AndroidManifest.xml'
            java.srcDirs = ['src']
            resources.srcDirs = ['src']
            aidl.srcDirs = ['src']
            renderscript.srcDirs = ['src']
            res.srcDirs = ['res']
            assets.srcDirs = ['assets']
        }

        instrumentTest.setRoot('tests')

        packagingOptions {
            exclude 'LICENSE.txt'
        }
    }
}

Running tests

A simple

gradle :core:test

should work, try changing the assertTrue(true) to assertTrue(false) in core/src/test/java/org/package/name/WorkingTest.java

Let me know if you have any other problems, I might add them here to help other. Enjoy :D