Build test helpers that point to hpc-htools

Instead of using just shell constructs to run hpc-htools correctly
(i.e. HTOOLS=role htools/hpc-htools …), let's add some shell fragments
that do this for us.

This will ease the running of tests directly.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>