Make test element selection more specific to fix constantly failing test.

Fixes #2051.

The problem was that there were two `.note-text` elements matching `.files [id^=diff]:nth-child(2) .note-text`, one for the new comment, and one for the (currently hidden) new comment form. The latter one was selected but currently invisible, triggering the following error in poltergeist:

> The element you are trying to interact with is either not part of the DOM, or is not currently visible on the page (perhaps display: none is set). It's possible the element has been replaced by another element and you meant to interact with the new element. If so you need to do a new 'find' in order to get a reference to the new element.

This caused the test to fail.

See merge request !1592