Replaced skipIf() with markTestSkipped()

http://api.cakephp.org/2.2/source-class-CakeTestCase.html#103-115
Party reconstructed testSubQueryEmptyCondition()

The test was very similar to testSubQueryCondition().
In the past this test looked different:
https://github.com/CakeDC/search/commit/784a1ca12a4bc87c2c0cc3ba2b5cdd92b3b484de#diff-6dd7f6dbf96b52929673d36d7e558781R306

Somebody with more in depth knowledge should have a look.