Cleanup create flavor tests

In preparation for some refactoring and additional features to the create
flavor method, this patch cleanups the existing tests by:

* Moving flavor create tests out to their own class

* Adding more coverage around boundary conditions

* Making tests more orthogonal (not testing that `root_gb` was set correctly in
  multiple tests, for example)

Change-Id: I82acadc19d2713406c3be01a104ccd64377098ed
Merge "Cleanup create flavor tests"