Gradle Android project with tests in dedicated subproject

lib_build.gradle

// Lib build file

android {
    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

root_build.gradle

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        google()
        jcenter()
        
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.5.2'
        
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

subprojects {
    apply plugin: 'com.android.library'

    android {
        compileSdkVersion 27

        defaultConfig {
            minSdkVersion 16
            targetSdkVersion 27
            versionCode 1
            versionName "1.0"

            testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        }

        compileOptions {
            sourceCompatibility 1.8
            targetCompatibility 1.8
        }
    }
    
    dependencies {
        implementation 'com.android.support:appcompat-v7:27.1.1'
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

test_build.gradle

// Tests build file

android {
    defaultConfig {
        multiDexEnabled true
    }
}

dependencies {
    implementation project(path: ':lib')

    testImplementation 'junit:junit:4.12'

    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    androidTestImplementation group: 'org.mockito', name: 'mockito-android', version: '3.1.0'
    androidTestImplementation group: 'org.assertj', name: 'assertj-core', version: '3.14.0'
}