Diablo and Hellfire have different palettes in `ui_art\black.pcx`.
We now ship our own tiny `black.pcx` that takes priority over the
other ones. This `black.pcx` had the Diablo palette which resulted in an
incorrect palette in the Settings screen.
Split our own `black.pcx` file into `black_diablo.pcx` and
`black_hellfire.pcx` and load the correct one depending on the mode.
1. Adds a `libdevilution_so` target when tests are enabled.
2. Each test file is now a separate binary target linked against `libdevilutionx_so` (can now run tests in parallel).
3. Tests are now defined in a separate `test/CMakeLists.txt` file.
4. Building the tests is now controlled by the standard `BUILD_TESTING` option (defined by CTest).
5. Tests are now built by default.
6. On CI, test errors are now reported.
Also:
* `.clang-format`: Enable SortIncludes in tests
* `path_test.cpp`: Fix -Wsign-compare