test: Migrate unit tests to gtest, integrate with CI (#2144)

scripts/register_unit_tests_target.py

@@ -0,0 +1,28 @@
+"""
+PlatformIO post-load script: register a `unit-tests` custom target so
+`pio run -t unit-tests` builds and runs the host gtest suites under test/.
+
+The target shells out to CMake/CTest; the gtest framework is fetched and
+the suites are built outside the PlatformIO/ESP-IDF toolchain (this is a
+host build, not a firmware build).
+"""
+
+import os
+
+Import("env")  # noqa: F821  -- provided by PlatformIO at script load
+
+PROJECT_DIR = env["PROJECT_DIR"]  # noqa: F821
+BUILD_DIR = os.path.join(PROJECT_DIR, "build", "test")
+TEST_SRC_DIR = os.path.join(PROJECT_DIR, "test")
+
+env.AddCustomTarget(  # noqa: F821
+    name="unit-tests",
+    dependencies=None,
+    actions=[
+        f'cmake -S "{TEST_SRC_DIR}" -B "{BUILD_DIR}" -DCMAKE_BUILD_TYPE=Release',
+        f'cmake --build "{BUILD_DIR}"',
+        f'ctest --test-dir "{BUILD_DIR}" --output-on-failure -j',
+    ],
+    title="Host unit tests",
+    description="Build and run gtest suites in test/ via CMake/CTest",
+)