Extend hroller test to also verify tag-based node selection

While the multiple-tags test was added to verify that coloring is done
only after node selection (otherwise it wouldn't be possible to get in
both cases a single reboot group), it can easily be extended to also
verify that the correct nodes are selected by --node-tags.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>