Fix mac-then-encrypt test with enable-tls1_3

Commit b3618f44

 added a test for mac-then-encrypt. However the test fails
when running with "enable-tls1_3". The problem is that the test creates a
connection, which ends up being TLSv1.3. However it also restricts the
ciphers to a single mac-then-encrypt ciphersuite that is not TLSv1.3
compatible so the connection aborts and the test fails. Mac-then-encrypt
is not relevant to TLSv1.3, so the test should disable that protocol
version.

Reviewed-by: Emilia Käsper <emilia@openssl.org>