Fix relative imports in manifold_learning/regression/tests.

Add a __init__.py file to the tests directory to enable relative imports.

From: Fabian Pedregosa <fabian.pedregosa@inria.fr>

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@373 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8