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c: validate PKCS#7 padding manually in the macOS backend

`test_returns_null_on_undecryptable_ciphertext` flips one byte of a
known-good ciphertext and expects decryption to fail. On Linux/OpenSSL
and Windows/BCrypt this works because both APIs validate PKCS#7
padding strictly during decrypt-final and report failure. On
macOS/CommonCrypto with `kCCOptionPKCS7Padding` the API documents
that it *strips* the padding without *validating* it - so a tampered
last block whose decrypted last byte happens to fall in [1, 16] is
silently truncated, the decrypt returns success, and the test fails.

Switch the macOS backend to call `CCCrypt` without
`kCCOptionPKCS7Padding`, then validate the padding by hand: the last
byte must be in [1, kCCBlockSizeAES128] and every padding byte must
equal that value, matching the strict PKCS#7 contract the other two
backends enforce. Behaviour for valid ciphertexts is unchanged; the
behaviour for tampered ciphertexts now matches Linux/Windows.

Signed-off-by: Andrew Udvare <audvare@gmail.com>

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