Remove duplicated test files

Even if they used to test different things in the past, they are
now identical to other files.

Closes #11496
auto merge of #15106 : Sawyer47/rust/rm-duplicated-tests, r=alexcrichton

Even if they used to test different things in the past, they are
now identical to other files.

Closes #11496
Add the Guide, add warning to tutorial.

In line with what @brson, @cmr, @nikomatsakis and I discussed this morning, my
redux of the tutorial will be implemented as the Guide. This way, I can work in
small iterations, rather than dropping a huge PR, which is hard to review.  In
addition, the community can observe my work as I'm doing it.

This adds a note in line with [this comment][reddit] that clarifies the state
of the tutorial, and the community's involvement with it.

[reddit]: http://www.reddit.com/r/rust/comments/28bew8/rusts_documentation_is_about_to_drastically/ci9c98k