test: unit tests of the simulation core, run as part of the build

eu07_unit_tests: C++ tests (doctest) of the simulation, which set up the
conditions, run the code and check the outcome. No game data, no window.

- the code under test is the core library (eu07_core), the very objects
  the game links, built with the game's settings; engine_stubs.cpp stands
  in for the logs, settings and scenery layers the library reaches for.
  python gets linked with the settings but isn't needed to run the tests
- test vehicles are JSON definitions in tests/unit/data, with the sections
  and keys of the game's vehicle definitions (tables as "rows", _keys are
  comments), changed by the tests where needed and loaded from memory by
  the game's own loader; an unknown section is an error rather than
  silently skipped
- test::vehicle creates the mover the way TDynamicObject::Init() does and
  steps it the way the simulation loop does, on a straight track of given
  friction and gradient, optionally holding the brake pipe pressure the
  way a coupled locomotive would
- the random numbers of the simulation are reseeded before every test
  case, so a test gives the same result alone (IDE) or among the others
- building the game runs the tests at the end, once the game is linked,
  and prints a one line summary ("Unit tests: 11/11 passed (80 checks)").
  A failure prints the full report and fails the build, in CI and locally
  alike, and removes the game binary so the next build reruns them. A
  change of the tests alone relinks the game so they run (Ninja and
  Makefile generators). -DWITH_TESTS=OFF leaves them out
- first tests: definitions reach the vehicle, KE service application and
  release, deeper reduction brakes harder, every implemented distributor
  valve charges/applies/releases, service braking stops a moving wagon

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/CMakeLists.txt

@@ -0,0 +1,31 @@
+# Unit tests of the simulation, built and run without the game data: the test vehicles are JSON
+# definitions in data/ (see vehicle.h), loaded from memory by the game's own vehicle loader. The code
+# under test is the game's core library (eu07_core), the very objects the game links; the rest of the
+# engine it reaches for is replaced with stubs (engine_stubs.cpp). They run as part of the build,
+# from the IDE, or with ctest.
+
+set(UNIT_TESTS_NAME eu07_unit_tests)
+
+# a new test_*.cpp file or data/*.json definition is picked up on its own
+file(GLOB UNIT_TEST_CASES CONFIGURE_DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/test_*.cpp")
+file(GLOB UNIT_TEST_DATA CONFIGURE_DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/data/*.json")
+
+add_executable(${UNIT_TESTS_NAME}
+	main.cpp
+	vehicle.cpp
+	engine_stubs.cpp
+	${UNIT_TEST_CASES})
+
+# the core library brings the game's build settings along
+target_link_libraries(${UNIT_TESTS_NAME} PRIVATE eu07_core)
+target_include_directories(${UNIT_TESTS_NAME} PRIVATE "${DEPS_DIR}/doctest")
+target_compile_definitions(${UNIT_TESTS_NAME} PRIVATE EU07_UNIT_TESTS_DATA="${CMAKE_CURRENT_SOURCE_DIR}/data")
+if (MSVC)
+	target_compile_options(${UNIT_TESTS_NAME} PRIVATE /bigobj)
+endif()
+set_target_properties(${UNIT_TESTS_NAME} PROPERTIES FOLDER "tests")
+
+add_test(NAME ${UNIT_TESTS_NAME} COMMAND ${UNIT_TESTS_NAME})
+
+# for running the tests at the end of building the game, see the main CMakeLists.txt
+set(UNIT_TEST_DATA ${UNIT_TEST_DATA} PARENT_SCOPE)