✅ 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.

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

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)