test_sslcbcpadding only makes sense <TLS1.3

We may get failures if we run it in TLS1.3, and it makes no sense anyway
so force TLS1.2
Reviewed-by: NRich Salz <rsalz@openssl.org>

test/recipes/70-test_sslcbcpadding.t

@@ -48,6 +48,7 @@ ok(TLSProxy::Message->success(), "Maximally-padded record test");
 # Test that invalid padding is rejected.
 foreach my $offset (@test_offsets) {
     $proxy->clear();
+    $proxy->serverflags("-tls1_2");
     $bad_padding_offset = $offset;
     $proxy->start();
     ok(TLSProxy::Message->fail(), "Invalid padding byte $bad_padding_offset");