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/main.cpp

@@ -0,0 +1,39 @@
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include "stdafx.h"
+#include "doctest.h"
+
+#include "utilities/Globals.h"
+
+namespace {
+
+// the simulation draws random numbers (brake delays, pressures, wheel damage) from engines shared by
+// everything. reseeding them before every run of a test case, subcases included, gives each test the
+// same numbers whether it runs alone from the IDE or among all the others
+class deterministic_random : public doctest::IReporter {
+public:
+	explicit deterministic_random(doctest::ContextOptions const &) {}
+
+	void test_case_start(doctest::TestCaseData const &) override { reseed(); }
+	void test_case_reenter(doctest::TestCaseData const &) override { reseed(); }
+
+	void report_query(doctest::QueryData const &) override {}
+	void test_run_start() override {}
+	void test_run_end(doctest::TestRunStats const &) override {}
+	void test_case_end(doctest::CurrentTestCaseStats const &) override {}
+	void test_case_exception(doctest::TestCaseException const &) override {}
+	void subcase_start(doctest::SubcaseSignature const &) override {}
+	void subcase_end() override {}
+	void log_assert(doctest::AssertData const &) override {}
+	void log_message(doctest::MessageData const &) override {}
+	void test_case_skipped(doctest::TestCaseData const &) override {}
+
+private:
+	static void reseed() {
+		Global.random_engine.seed(1);
+		Global.local_random_engine.seed(1);
+	}
+};
+
+} // namespace
+
+DOCTEST_REGISTER_LISTENER("deterministic_random", 1, deterministic_random);