Clean up following new SNI tests

- Only send SNI in SNI tests. This allows us to test handshakes without
  the SNI extension as well.
- Move all handshake-specific machinery to handshake_helper.c
- Use enum types to represent the enum everywhere
  (Resorting to plain ints can end in sign mismatch when the enum is
  represented by an unsigned type.)

Reviewed-by: Rich Salz <rsalz@openssl.org>