add tests for fail-safe map of ZDungeon
move copy-constructor, operator= away from ZDungeon's public interface
add tests for ZDungeon::CellIsEmpty
add tests for ZDungeon::CellIsSolid
add sample tests for ZDungeon::GetStairsUp
add ASSERT_POSITION_EQ macro
move #include of gtest.h to PCH for cpp-files of projectzTest