Remove dir prefix from filename of tests under dir.

Rename refactoring only commit. E.g.:

- rename `features/project/commits/commit_comments.feature` to `comments.feature`
- rename `features/steps/project/project_wiki.rb` to `wiki.rb`
- rename `features/steps/snippets/user_snippets.rb` to `user.rb`

**Advantages**

- its DRYer: before we repeated the directory name in the files it contain.

- we type less: every time I want to select a `project_X` file I have to do `pro<tab>X<tab>`. This lets us do only `X<tab>` instead.

- it is more uniform: certain tests were already breaking the dir prefix "rule" before this commit, e.g.:

    - `features/steps/project/deploy_keys.rb` already did not have the prefix before.

    - under steps, most files already don't contain the dir prefix. This removes it from the few that do.

**Disadvantages**

The only downside I can see is that if your editor only shows basenames on tabs, you can't tell some files apart. However:

- when working on a branch, it is likely that you will only touch files under a single directory.
- if you use a good editor (e.g. Vim) you can configure it to show `parent/basename` on tabs instead.

Therefore, I feel that the advantages outweight the disadvantages.

**doubts**

- Exception was made for files such as `project/project.rb`, which are unchanged. What if we rename those as `main.rb` or `shared.rb` to be DRYer?

- What if we rename all suffixes classes like `PublicProjects < Spinach::FeatureSteps` to prefix classes like `ProjectsPublic`, which is already the case for the majority of tests? This would make this refactoring even neater.