Make crypto_sign() test code more explicit.
crypto_sign() doesn't just need the secret key. The public key has to follow.
Which is why the test vectors are laid out in this order.
But this can confuse static analysis, as well as people looking at the test
in order to better understand how crypto_sign() works.
So, just copy the sk and the pk into a dedicated buffer, for clarity.
test/default/sign.c: make the test vectors static