Fix SSL tests scripts with recent OpenSSL server with Diffie-Hellman

Our interoperability tests fail with a recent OpenSSL server. The
reason is that they force 1024-bit Diffie-Hellman parameters, which
recent OpenSSL (e.g. 1.1.1f on Ubuntu 20.04) reject:
```
140072814650688:error:1408518A:SSL routines:ssl3_ctx_ctrl:dh key too small:../ssl/s3_lib.c:3782:
```

We've been passing custom DH parameters since
6195767554da332e9f81e6510b07f7565ff8a538 because OpenSSL <=1.0.2a
requires it. This is only concerns the version we use as
OPENSSL_LEGACY. So only use custom DH parameters for that version. In
compat.sh, use it based on the observed version of $OPENSSL_CMD.

This way, ssl-opt.sh and compat.sh work (barring other issues) for all
our reference versions of OpenSSL as well as for a modern system OpenSSL.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index db898cf..33ae4e4 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -1197,7 +1197,7 @@
 P_SRV="$P_SRV server_addr=127.0.0.1 server_port=$SRV_PORT"
 P_CLI="$P_CLI server_addr=127.0.0.1 server_port=+SRV_PORT"
 P_PXY="$P_PXY server_addr=127.0.0.1 server_port=$SRV_PORT listen_addr=127.0.0.1 listen_port=$PXY_PORT ${SEED:+"seed=$SEED"}"
-O_SRV="$O_SRV -accept $SRV_PORT -dhparam data_files/dhparams.pem"
+O_SRV="$O_SRV -accept $SRV_PORT"
 O_CLI="$O_CLI -connect localhost:+SRV_PORT"
 G_SRV="$G_SRV -p $SRV_PORT"
 G_CLI="$G_CLI -p +SRV_PORT"