Migrate unit tests to PlatformIO Unity

Replace the legacy tests/ tree (AUnit, EpoxyDuino, Make, AUniter) with
PlatformIO Unity suites under test/, shared helpers, and platformio.ini.
Remove AUnitHelpers and drop the AUnit include from ArduinoHA test builds.
Refresh docs to Markdown-only and document the test workflow in CHANGELOG.

platformio.ini

@@ -0,0 +1,38 @@
+; PlatformIO — Unity tests for home-assistant-integration (ArduinoHA)
+;
+; Full suite:
+;   pio test -e esp8266
+;   pio test -e esp32
+;
+; Single suite (examples):
+;   pio test -e esp8266 --filter test_utils_serializers
+;   pio test -e esp8266 --filter test_mqtt_core
+;   pio test -e esp8266 --filter test_entities_basic
+;   pio test -e esp8266 --filter test_entities_numeric
+;   pio test -e esp8266 --filter test_entities_extended
+;   pio test -e esp8266 --filter test_entities_misc
+;
+; See: https://docs.platformio.org/en/latest/advanced/unit-testing/structure/hierarchy.html
+
+[platformio]
+default_envs = esp8266
+
+[common]
+framework = arduino
+test_framework = unity
+test_build_src = yes
+monitor_speed = 115200
+build_flags = -DARDUINOHA_TEST
+lib_deps = knolleary/PubSubClient@^2.8.0
+
+[env:esp8266]
+extends = common
+platform = espressif8266
+board = d1_mini
+
+[env:esp32]
+extends = common
+platform = espressif32
+board = esp32dev
+build_unflags = -std=gnu++11
+build_flags = ${common.build_flags} -std=gnu++14