Add missing unit tests for some PXE drivers
Add missing unit tests for some PXE drivers

Some of the pxe_* drivers don't have unit tests, while some others have.
Unit tests need to be available for other drivers as well.

Change-Id: I6be2d65c00e575bb0df7309f0e891fdf37cb731f
Closes-bug: #1512966

pshchelo/ironic,openstack/ironic,ionutbalutoiu/ironic,ionutbalutoiu/ironic,dims/ironic,devananda/ironic,SauloAislan/ironic,SauloAislan/ironic,hpproliant/ironic,dims/ironic,pshchelo/ironic,openstack/ironic,NaohiroTamura/ironic,bacaldwell/ironic,NaohiroTamura/ironic,bacaldwell/ironic