Feature: opt-in switch to flag untestable variants

[blex-max]

Occasionaly hairpin2 encounters a variant it cannot test on any flags. This is almost always because it detects no supporting reads. This situation is, so far, always associated with something strange going on with the variant in question. While it's outside the scope of the hairpin2 tests, some users have requested that this "no support" situation is added as an opt in additional flag. This should be easy enough to add via the execution config