Wait till the counter has the desired value, instead of assuming that
the counter already has the expected value. This fixes the tests on
slow browsers.
Furthermore, the wait isn't needed any more, now that the counter
queries are race ondition proof.
The waitForIconMarkReadInvisible was the wrong wait function here, since
the whole article will be hidden instead of the read icon. This could
lead into race condition related errors if the article is hidden before
the waitForIconMarkRead() functions runs. The article variable that is
used to address the child read icon can refer to an (DOM) object which
doesn't exist any longer => StaleElementReferenceException.
The correct wait function in such a case would be waitForArticleInvisible().
