Set up build.gradle Junit Android tests

I am begineer in Android test. I am trying execute a Junit test, but it crashes when I load a native library.

This is my build.gradle file:

apply plugin: 'com.android.library'

android {
    compileSdkVersion 24
    buildToolsVersion "24.0.0"

    defaultConfig {
        minSdkVersion 18
        targetSdkVersion 24
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}


dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    compile 'com.android.support:appcompat-v7:24.0.0'
    // Unit testing dependencies
    testCompile 'junit:junit:4.12'
    // Set this dependency if you want to use the Hamcrest matcher library
    testCompile 'org.hamcrest:hamcrest-library:1.3'
    // more stuff, e.g., Mockito
    testCompile 'org.mockito:mockito-core:1.10.19'
}

repositories {
    flatDir {
        dirs 'libs'
    }
}

And when I do

static
    {
        System.loadLibrary("a");
    }

I obtain java.lang.UnsatisfiedLinkError: no a in java.library.path

My test is in "src/test/java" and my native library in "src/test/jniLibs". I suppose than I have to specify Jni libraries path in build.gradle, but how?

Thanks