Make test comments more consistent.

tests/TestCase/Cache/Engine/ApcEngineTest.php

@@ -288,7 +288,7 @@ class ApcEngineTest extends TestCase
     }
 
     /**
-     * Test that failed add write return false.
+     * Test add
      *
      * @return void
      */

tests/TestCase/Cache/Engine/MemcachedEngineTest.php

@@ -918,7 +918,7 @@ class MemcachedEngineTest extends TestCase
     }
 
     /**
-     * Test that failed add write return false.
+     * Test add
      *
      * @return void
      */

tests/TestCase/Cache/Engine/RedisEngineTest.php

@@ -412,7 +412,7 @@ class RedisEngineTest extends TestCase
     }
 
     /**
-     * Test that failed add write return false.
+     * Test add
      *
      * @return void
      */