Extend hroller shell tests

Currently only --help and --version are tested; the patch extends this
with testing parsing of an empty cluster, of a cluster with one group,
and other command line behaviours.

It also adds testing for --help-completion for all htools binaries.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>