Set constant visibility in tests to private where possible.

Author: derrabus

Tests/Session/Storage/Handler/MemcachedSessionHandlerTest.php

@@ -20,8 +20,8 @@
  */
 class MemcachedSessionHandlerTest extends TestCase
 {
-    const PREFIX = 'prefix_';
-    const TTL = 1000;
+    private const PREFIX = 'prefix_';
+    private const TTL = 1000;
 
     /**
      * @var MemcachedSessionHandler