test_maps: don't print stack traceback on failure, only message
test_maps: modularize for correct test suite behavior, move `loadLevels()` from tests to `suite_setup`

Lunatest's `fail` and `assert_*` are not supported in `suite_setup`, replace with standard `error` and `assert`.
Rename tests; `_maps` infix is not needed, tests are identifiable through test suite name (`test_maps`)