With this, we no longer require `std::filesystem` support to create the
save file directory when it does not exist.
Note that by default the save file directory exists because it is
created by SDL, so this is mostly useful on platforms that override the
save directory or use `UNPACKED_SAVES`.
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
Gleb Mazovetskiy