Expand test platforms.

Closes #2996055

src/Driver/DriverBase.php

@@ -243,8 +243,9 @@ abstract class DriverBase implements DrupalMemcacheInterface {
    */
   public function getServers() {
     $memcache_servers = \Drupal::configFactory()->getEditable('memcache.settings')->get('memcache_servers');
+    $host = getenv('MEMCACHED_HOST') ?: '127.0.0.1:11211';
     if (!isset($memcache_servers)) {
-      $memcache_servers = ['127.0.0.1:11211' => 'default'];
+      $memcache_servers = [$host => 'default'];
     }
 
     return $memcache_servers;