Add signing tests with 528-bit and 520-bit RSA keys with SHA-512. These
selections of key and hash size should lead to an error returned, as
there is not enough room for our chosen minimum salt size of two bytes
less than the hash size. These test the boundary around an available
salt length of 0 or -1 bytes.

The RSA keys were generated with OpenSSL 1.1.1-pre8.

    $ openssl genrsa 520
    Generating RSA private key, 520 bit long modulus (2 primes)
    .............++++++++++++
    .................++++++++++++
    e is 65537 (0x010001)
    -----BEGIN RSA PRIVATE KEY-----
    MIIBPwIBAAJCANWgb4bludh0KFQBZcqWb6iJOmLipZ0L/XYXeAuwOfkWWjc6jhGd
    B2b43lVnEPM/ZwGRU7rYIjd155fUUdSCBvO/AgMBAAECQgDOMq+zy6XZEjWi8D5q
    j05zpRGgRRiKP/qEtB6BWbZ7gUV9DDgZhD4FFsqfanwjWNG52LkM9D1OQmUOtGGq
    a9COwQIhD+6l9iIPrCkblQjsK6jtKB6zmu5NXcaTJUEGgW68cA7PAiENaJGHhcOq
    /jHqqi2NgVbc5kWUD/dzSkVzN6Ub0AvIiBECIQIeL2Gw1XSFYm1Fal/DbQNQUX/e
    /dnhc94X7s118wbScQIhAMPVgbDc//VurZ+155vYc9PjZlYe3QIAwlkLX3HYKkGx
    AiEND8ndKyhkc8jLGlh8aRP8r03zpDIiZNKqCKiijMWVRYQ=
    -----END RSA PRIVATE KEY-----

    $ openssl genrsa 528
    Generating RSA private key, 528 bit long modulus (2 primes)
    .........++++++++++++
    ....++++++++++++
    e is 65537 (0x010001)
    -----BEGIN RSA PRIVATE KEY-----
    MIIBQgIBAAJDAKJVTrpxW/ZuXs3z1tcY4+XZB+hmbnv1p2tBUQbgTrgn7EyyGZz/
    ZkkdRUGQggWapbVLDPXu9EQ0AvMEfAsObwJQgQIDAQABAkJhHVXvFjglElxnK7Rg
    lERq0k73yqfYQts4wCegTHrrkv3HzqWQVVi29mGLSXTqoQ45gzWZ5Ru5NKjkTjko
    YtWWIVECIgDScqoo7SCFrG3zwFxnGe7V3rYYr6LkykpvczC0MK1IZy0CIgDFeINr
    qycUXbndZvF0cLYtSmEA+MoN7fRX7jY5w7lZYyUCIUxyiOurEDhe5eY5B5gQbJlW
    ePHIw7S244lO3+9lC12U1QIhWgzQ8YKFObZcEejl5xGXIiQvBEBv89Y1fPu2YrUs
    iuS5AiFE64NJs8iI+zZxp72esKHPXq/chJ1BvhHsXI0y1OBK8m8=
    -----END RSA PRIVATE KEY-----