Figure out a file and directory structure for tests

Problem/Motivation

Because of the size of Drupal.org, the number of people we'd like working on tests, and the fact that nearly all of us are just learning, it would be nice to have some separation between major feature sets in the context files so that each area is relatively self-contained.

Proposed resolution

(description of the proposed solution, the rationale behind it, and workarounds for people who cannot use the patch)
I made a commit to the eliza411 topic branch that uses Behat subcontexts to accomplish this:

• FeatureSet.php includes resources for all subcontexts
• Each subcontext initializes the test environment independently (and is responsible for tearing it down, too).

I can imagine that once we have tests written and a better understanding of all the parts, we may want to initialize the test environment globally, but it seemed like the containment might be beneficial during this early stage.

For the time being, I imagine each team having its own context file. As more tests are written, we can consider a more elaborate directory structure as needed.

For now, to set up a new feature set:

1. Create a file called FeaturesetnameContext.php in features/bootstrap (e.g. GitContext.php)
2. Edit features/bootstrap/FeatureContext.php and call the new file in the contstructor, for example: $this->useContext('git_context', new GitContext($parameters));

Remaining tasks

(reviews needed, tests to be written or run, documentation to be written, etc.)