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
obligaron