Add test case for D2 red mine objects

Reportedly we have a bug when loading levels with the polygonal red mine objects seen in D2 level 3. This wasn't caught by unit tests because the D2 test level doesn't have any of those; we need to fix that.
May require adjusting some other test cases, if I recall correctly Rl2Tests uses hard-coded numbers in a few cases that could change if the game data is updated.