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() {
// Save the original $_GET to be restored later.
$this->GET = $_GET;
parent::setUp();
}