Test mac-then-encrypt

Verify that the encrypt-then-mac negotiation is handled
correctly. Additionally, when compiled with no-asm, this test ensures
coverage for the constant-time MAC copying code in
ssl3_cbc_copy_mac. The proxy-based CBC padding test covers that as
well but it's nevertheless better to have an explicit handshake test
for mac-then-encrypt.

Reviewed-by: Andy Polyakov <appro@openssl.org>