QCHelper: add support for defining HUnit test cases

This will allow us to use exactly the same method as for defining and
using QuickCheck properties. The differentiation is based on name,
which is not very nice but is the same method used in
test-framework-th, for example (so we will be able to switch
trivially).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>