✅ 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>

buildroot/share/PlatformIO/scripts/collect-code-tests.py

@@ -0,0 +1,59 @@
+#
+# collect-code-tests.py
+# Convenience script to collect all code tests. Used by env:linux_native_test in native.ini.
+#
+
+import pioutil
+if pioutil.is_pio_build():
+
+    import os, re
+    Import("env")
+    Import("projenv")
+
+    os.environ['PATH'] = f"./buildroot/bin/:./buildroot/tests/:{os.environ['PATH']}"
+
+    def collect_test_suites():
+        """Get all the test suites"""
+        from pathlib import Path
+        return sorted(list(Path("./test").glob("*.ini")))
+
+    def register_test_suites():
+        """Register all the test suites"""
+        targets = []
+        test_suites = collect_test_suites()
+        for path in test_suites:
+            name = re.sub(r'^\d+-|\.ini$', '', path.name)
+            targets += [name];
+
+            env.AddCustomTarget(
+                name = f"marlin_{name}",
+                dependencies = None,
+                actions = [
+                    f"echo ====== Configuring for marlin_{name} ======",
+                    "restore_configs",
+                    f"cp -f {path} ./Marlin/config.ini",
+                    "python ./buildroot/share/PlatformIO/scripts/configuration.py",
+                    f"platformio test -e linux_native_test -f {name}",
+                    "restore_configs",
+                ],
+                title = "Marlin: {}".format(name.lower().title().replace("_", " ")),
+                description = (
+                    f"Run a Marlin test suite, with the appropriate configuration, "
+                    f"that sits in {path}"
+                )
+            )
+
+        env.AddCustomTarget(
+            name = "test-marlin",
+            dependencies = None,
+            actions = [
+                f"platformio run -t marlin_{name} -e linux_native_test"
+                for name in targets
+            ],
+            title = "Marlin: Test all code test suites",
+            description = (
+                f"Run all Marlin code test suites ({len(targets)} found)."
+            ),
+        )
+
+    register_test_suites()

buildroot/share/PlatformIO/scripts/preflight-checks.py

@@ -71,7 +71,9 @@ if pioutil.is_pio_build():
         config = env.GetProjectConfig()
         result = check_envs("env:"+build_env, board_envs, config)
 
-        if not result:
+        # Make sure board is compatible with the build environment. Skip for _test,
+        # since the board is manipulated as each unit test is executed.
+        if not result and build_env != "linux_native_test":
             err = "Error: Build environment '%s' is incompatible with %s. Use one of these environments: %s" % \
                   ( build_env, motherboard, ", ".join([ e[4:] for e in board_envs if e.startswith("env:") ]) )
             raise SystemExit(err)