Add unit tests for CryptoKey

[wheatear-dev]

Further to #43440, this Pull Request adds several unit tests for CryptoKey, as a second attempt after the related #88988 .

CryptoKey Load and save a .key file.

These concern a CryptoKey instance that has loaded in the attached keys, under tests/data/crypto/:

• load() a private/public key is respected by is_public_only()
• default save() leads to identical file contents for tests/data/crypto/in.key and tests/data/crypto/in.pub, since they are the same key
• save() with public_only set to true leads to identical contents as the separately prepared public key, namely tests/data/crypto/in.pub

Additionally, DOCTEST_CONFIG_NO_EXCEPTIONS_BUT_WITH_ALL_ASSERTS was added to the CPPDEFINES of the mbedtls module, if both tests are enabled and exceptions are disabled. This was so that the REQUIRE assert could be used.

[AThousandShips]

Looks good to me

CC @Faless

[Faless]

LGTM

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉