Break down tests into categories.

Break down tests into categories to improve maintainability and test granularity. This makes it easier to execute unit tests in CI/CD pipelines.

Closes #1054.

test_apps/main/CMakeLists.txt

@@ -1,3 +1,20 @@
-idf_component_register(SRCS "test_relay_chn.c"
-                       INCLUDE_DIRS "."
-                       REQUIRES unity relay_chn)
+# === These files must be included in any case ===
+set(srcs "test_common.c"
+         "test_app_main.c"
+         "test_relay_chn_core.c"
+         "test_relay_chn_listener.c")
+
+if(CONFIG_RELAY_CHN_ENABLE_TILTING)
+    list(APPEND srcs "test_relay_chn_tilt.c")
+endif()
+
+message(STATUS "srcs=${srcs}")
+
+# In order for the cases defined by `TEST_CASE` to be linked into the final elf,
+# the component can be registered as WHOLE_ARCHIVE
+idf_component_register(
+    SRCS ${srcs}
+    INCLUDE_DIRS "."
+    REQUIRES unity relay_chn
+    WHOLE_ARCHIVE
+)