kdf: Restructure KDF test vectors

* tests/t-kdf.c: Restructure KDF test vectors to allow easy addition
  new vectors. Also remove some ugly C code like goto again.

[jk: fixed commit changelog cipher/kdf.c -> tests/t-kdf.c]

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>