Some more changes to Makefile.am for htools

I duplicate the BINARY= rule in the ghc invocation in order to be able
to silence the if, which was confusing.

Additionally, a new target for running just the htools unit-tests is
provided.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>