New x/crypto release breaks ssh tests ("crypto/rsa: message too long for RSA public key size")

[gibmat]

Recently, the x/crypto library switched to SHA-512 for the default RSA certificate signature, which doesn't fit in 512 bits (https://cs.opensource.google/go/x/crypto/+/b4de73f9ece8163b492578e101e4ef8923ac2c5c). This causes several of the ssh tests to fail with the error "crypto/rsa: message too long for RSA public key size".

Updating generateRSAKey() in ssh/generate_test.go to use a 1024 bit key size fixes the problem.