Add tests for Memcached to ensure its counters have TTLs

Because memcached doesn't allow cache keys to be incremented/decremented
without a default value (unless you use the binary protocol), we don't
need an explict TTL set as the key will have a TTL set when it is
initially written.

Refs #10786

tests/TestCase/Cache/Engine/MemcachedEngineTest.php

@@ -722,6 +722,26 @@ public function testIncrement()
         Cache::delete('test_increment', 'memcached');
     }
 
+    /**
+     * Test that increment and decrement set ttls.
+     *
+     * @return void
+     */
+    public function testIncrementDecrementExpiring()
+    {
+        $this->_configCache(['duration' => 1]);
+        Cache::write('test_increment', 1, 'memcached');
+        Cache::write('test_decrement', 1, 'memcached');
+
+        $this->assertSame(2, Cache::increment('test_increment', 1, 'memcached'));
+        $this->assertSame(0, Cache::decrement('test_decrement', 1, 'memcached'));
+
+        sleep(1);
+
+        $this->assertFalse(Cache::read('test_increment', 'memcached'));
+        $this->assertFalse(Cache::read('test_decrement', 'memcached'));
+    }
+
     /**
      * test incrementing compressed keys
      *