Exception raising in TestEngine.verify()

This patch changes the interface of the TestEngine.verify() method. In the
previous version, it returned True if all tests passed and False otherwise.
This patch introduces a custom VerificationException, which is now thrown
by the verify() method in case of a failure, instead of returning False.

Blueprint test-engine

Change-Id: I0789d6c56f6952e3056e6dad022690eb90178e3d