A fairly extensive refactor of the mirror testing code. The exhaustive

A fairly extensive refactor of the mirror testing code. The exhaustive
lvconvert testing is now in its own test, t-lvconvert-mirror-basic ... it
doesn't do anything fancy but it does run lvconvert through a lot of
combinations.

I have also merged the remaining t-mirror-lvconvert tests into
t-lvconvert-mirror and abolished the former. The latter will be split again
later into more thematic divisions. (The previous split was rather arbitrary,
may I even say random...)