Adding basic 'file' feature, step defn, & class file

To improve the automated search testing, we'll need to
be able to check whether a particular file exists on the
target install.  Here's some bare-bones code to test MediaWiki's
behavior for when a file doesn't exist, both anon & logged-in.

Change-Id: I773483a0793b96b931e084ad74991a3a812db5b7