This adds a very powerful method to the TestProfile class,
group_manager. This group_manager method is a context_manager (it's used
with the 'with' statement), and is passed a test class and a flattened
group name (such as one returned by grouptools.join), and yields a
callable that is used to add tests to the profile.

This gives us a lot of advantages. First, it means that tests are added
in a context, so if we need to define data structures for adding tests
to a specific group (say a list of texture formats supported by a
specific version of GL), that data structure is defined in a nested
scope, so it cannot be accidentally be used in a different group.
Second, it means not passing a group around anymore, in fact, it creates
an abstraction on top of the group data structure so it doesn't matter
how we're representing it. Third the function itself is more flexible
than anything we've had before. It can either take an explicit name, or
it can call ' '.join() on the arguments to the Test, and use that as a
name, which can help make Test assignments much less verbose.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>