Sets up unit test environment.
Unlike Drupal\simpletest\WebTestBase::setUp(), UnitTestBase::setUp() does not install modules because tests are performed without accessing the database. Any required files must be explicitly included by the child class setUp() method.
Overrides UnitTestBase::setUp
function setUp() {
  parent::setUp();
  $this->variables = array(
    'foo' => 'bar',
    'baz' => array(
      'foo' => '42',
      'bar' => '23',
    ),
    'node' => new TwigReferenceObjectTest(42, 'test node'),
  );
}