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
protected function setUp() {
parent::setUp();
module_load_install('system');
$schema = system_schema();
db_create_table('key_value_expire', $schema['key_value_expire']);
}