Upon investigating an user report in #6091 that indicated that viewing
replays using the Perfect mod would also display the Sudden Death mod
icon despite Perfect being the more restrictive of the two, it turned
out that the logic of importing legacy scores was missing that corner
case. A similar case of Double Time/Nightcore mutual exclusion was
handled, but PF/SD was missed.
Add analogous handling of PF/SD legacy mods for all four rulesets,
and additionally cover a tiny fraction of all cases with unit tests.
The most problematic cases (NC+HD and PF+SD) are covered in all four
basic rulesets.
