✅ Add unit testing framework (#26948)

- Add a framework to build and execute unit tests for Marlin.
- Enable unit test execution as part of PR checks.

---------

Co-authored-by: Costas Basdekis <costas.basdekis@gmail.com>
Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

test/unit_tests.h

@@ -0,0 +1,73 @@
+/**
+ * Marlin 3D Printer Firmware
+ * Copyright (c) 2024 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ *
+ * Based on Sprinter and grbl.
+ * Copyright (c) 2011 Camiel Gubbels / Erik van der Zalm
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ */
+#pragma once
+
+#include <list>
+#include <string>
+#include <unity.h>
+
+// Include MarlinConfig so configurations are available to all tests
+#include "src/inc/MarlinConfig.h"
+
+/**
+ * Class that allows us to dynamically collect tests
+ */
+class MarlinTest {
+public:
+    MarlinTest(const std::string name, const void(*test)(), const int line);
+    /**
+     * Run the test via Unity
+     */
+    void run();
+
+    /**
+     * The name, a pointer to the function, and the line number. These are
+     * passed to the Unity test framework.
+     */
+    const std::string name;
+    const void(*test)();
+    const int line;
+};
+
+/**
+ * Internal macros used by MARLIN_TEST
+ */
+#define _MARLIN_TEST_CLASS_NAME(SUITE, NAME) MarlinTestClass_##SUITE##_##NAME
+#define _MARLIN_TEST_INSTANCE_NAME(SUITE, NAME) MarlinTestClass_##SUITE##_##NAME##_instance
+
+/**
+ * Macro to define a test. This will create a class with the test body and
+ * register it with the global list of tests.
+ *
+ * Usage:
+ * MARLIN_TEST(test_suite_name, test_name) {
+ *  // Test body
+ * }
+ */
+#define MARLIN_TEST(SUITE, NAME) \
+    class _MARLIN_TEST_CLASS_NAME(SUITE, NAME) : public MarlinTest { \
+    public: \
+        _MARLIN_TEST_CLASS_NAME(SUITE, NAME)() : MarlinTest(#NAME, (const void(*)())&TestBody, __LINE__) {} \
+        static void TestBody(); \
+    }; \
+    const _MARLIN_TEST_CLASS_NAME(SUITE, NAME) _MARLIN_TEST_INSTANCE_NAME(SUITE, NAME); \
+    void _MARLIN_TEST_CLASS_NAME(SUITE, NAME)::TestBody()