public function example() {
    return Drupal::config('my_module.job')->get('my_module.bucketName');
}

public function testExample() {
    $myModuleHelper = new MyModuleHelper();
        $expected_config = array(
            'bucketName' => 'test_bucket',
        );

    $this->assertEquals($expected_config, $myModuleHelper ->example());
}

public function setUp() {
    $this->config = $this->getMockBuilder('DrupalCoreConfigImmutableConfig')
        ->disableOriginalConstructor()
        ->getMock();

    $this->configFactory = $this->getMockBuilder('DrupalCoreConfigConfigFactory')
        ->disableOriginalConstructor()
        ->getMock();
    $this->configFactory->expects($this->any())
        ->method('get')
        ->with('my_module.job')
        ->willReturn($this->config);

    $this->container = new ContainerBuilder();
    $this->container->set('config.factory', $this->configFactory);
    Drupal::setContainer($this->container);
}

$this->configFactory = $this->getConfigFactoryStub([
  'mymodule.job' => ['bucketName' => 'test_bucket'],
]);

$this->configFactory = $this->getConfigFactoryStub([
  'mymodule.settings' => [
    'key' => [
      'subkey' => [
        'subsubkey' => 'value',
      ],
    ],
  ],
]);
$this->container = new ContainerBuilder();
$this->container->set('config.factory', $this->configFactory);
Drupal::setContainer($this->container);

Drupal::config('mymodule.settings')->get('key.subkey.subsubkey');