add ability to prevent assignment of guard, exit, and hsdir flags in testing networks

The new TestingDirAuthVoteExit, TestingDirAuthVoteGuard, and TestingDirAuthVoteHSDir options for testing networks are great. However, they do not allow configuration of nodes that should be in one group but not the other, or should be in none of the groups. I propose adding this feature.

A patch is attached.