test(blade): pin which rows of the mp table are blades

Add a unit test for the routine that selects the blades. The test lifts the
routine out of the plugin source, because the plugin needs a management node
to load.

The test gives the routine the row shapes that the writers of the mp table
make: a management module and its blades from xCAT::PPCdb::add_systemX and
the shipped mp template, a chassis that gives its hardware type, and one row
for each hardware type that lsslp writes. It also shows that the handler
reads the chassis attribute and that it stops before the arp table when the
mp table holds no blades.