reorganise test fixture directories

Files containing JSON encoded tests fixtures and their fixture loader
classes have been moved to the Xabbuh\XApi\CommonTests\Fixture namespace.
PHP fixture classes have been moved to the Xabbuh\XApi\DataFixtures
namespaces. Thus, they can be packaged separately to be reused in the
different xAPI packages.

The Common subtree cannot be pushed until the data fixtures package
has been created. Otherwise, the tests executed on this isolated subtree
would break since the data fixture classes could not be found.