Add forward-only support for disjoint groups

[mfeit-internet2]

Add a switch to prevent disjoint groups from generating reverse pairs. (I.e., [A, B] and [D, E] would make pairs for A-D, A-E, B-D and B-E but not D-A, D-B, E-A and E-B.)

The use case for this came up on SCinet, where I was having each test point test to its gateway and the gateways of all other test points:

I'd tried doing something clever with putting gateway information in _meta and generating tests based on that, but it looks like they were weeded out as duplicates.