tests/t-kdf: fix format string argument type mismatch

* tests/t-kdf.c (check_pbkdf2, check_scrypt): Change fprintf format
for plen from "%ld" to "%u" and cast plen to 'unsigned int'.

Fix compiler warning: "format ‘%ld’ expects argument of type ‘long int’, but
argument 3 has type ‘size_t’ {aka ‘unsigned int’}".

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