Use const keyword to define constants instead of define()

In Drupal 8 and later, constants should be defined using the const PHP language keyword (instead of define()), because it is better for performance:

/**
 * Minimum value of PHP memory_limit for SimpleTest.
 */
const SIMPLETEST_MINIMUM_PHP_MEMORY_LIMIT = '64M';

Note that const does not work with PHP expressions. define() should be used when defining a constant conditionally or with a non-literal value:

if (!defined('MAINTENANCE_MODE')) {
  define('MAINTENANCE_MODE', 'error');
}