tests: arduino-core-tests + test_config.h

[IFX-Anusha]

By creating this pull request you agree to the terms in CONTRIBUTING.md.
https://github.com/Infineon/.github/blob/master/CONTRIBUTING.md
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CONTRIBUTING.md also tells you what to expect in the PR process.

Description
Infineon/arduino-core-tests (with its nested Unity submodule) as a pinned submodule under arduino_core_tests.

Add test_config.h as a shared scaffold — one file, one #if defined(ARDUINO_) block per board — matching where the submodule's own Makefile expects it. No board blocks included here; those belong to the family branch that owns each board (see edge-arduino-core-tests-config for kit_pse84_ai).

README.md documents the convention and how to run a test via make -C extra/arduino_core_tests.

Verified: test_digitalio_single compiles, links, flashes, and runs correctly on real KIT-PSE84-AI hardware using this mechanism.

[jaenrig-ifx]

👍 nice!

[AkshayChandra-IFX]

Ran this on the board and it builds, flashes, and runs fine, with Unity reporting pass/fail as expected.
Sanity-checked the framework with the single-board time and random tests — both passed as expected (Unity reported them pass). test_digitalio_single mostly passed too (driven read/write and all the invalid-pin cases), but I had 3 failing cases there — the loopback checks reading HIGH instead of LOW, which looks like my jumper wiring. I'll debug into my setup before calling that one (digitalio)done.
Test framework looks fine 👌

[IFX-Anusha]

Ran this on the board and it builds, flashes, and runs fine, with Unity reporting pass/fail as expected. Sanity-checked the framework with the single-board time and random tests — both passed as expected (Unity reported them pass). test_digitalio_single mostly passed too (driven read/write and all the invalid-pin cases), but I had 3 failing cases there — the loopback checks reading HIGH instead of LOW, which looks like my jumper wiring. I'll debug into my setup before calling that one (digitalio)done. Test framework looks fine 👌

Then I will merge this and later any test specific/feature specific things can be added to edge-main/control-main