Enable 3DES with GnuTLS

With GnuTLS servers, 3DES-CBC cipher suites are enabled by default under our
GNUTLS_LEGACY (3.3.8), but disabled by default under more recent versions
including the one we use by default on the CI (3.4.6). Even modern
versions (I checked 3.7.2) support 3DES if explicitly enabled. So
unconditionally enable 3DES-CBC for GnuTLS.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/compat.sh b/tests/compat.sh
index b4a3638..1d8dfe4 100755
--- a/tests/compat.sh
+++ b/tests/compat.sh
@@ -1023,7 +1023,7 @@
     M_SERVER_ARGS="server_port=$PORT server_addr=0.0.0.0 force_version=$MODE arc4=1"
     O_SERVER_ARGS="-accept $PORT -cipher ALL,COMPLEMENTOFALL -$O_MODE"
     G_SERVER_ARGS="-p $PORT --http $G_MODE"
-    G_SERVER_PRIO="NORMAL:${G_PRIO_CCM}+ARCFOUR-128:+NULL:+MD5:+PSK:+DHE-PSK:+ECDHE-PSK:+SHA256:+SHA384:+RSA-PSK:-VERS-TLS-ALL:$G_PRIO_MODE"
+    G_SERVER_PRIO="NORMAL:${G_PRIO_CCM}+ARCFOUR-128:+3DES-CBC:+NULL:+MD5:+PSK:+DHE-PSK:+ECDHE-PSK:+SHA256:+SHA384:+RSA-PSK:-VERS-TLS-ALL:$G_PRIO_MODE"
 
     # The default prime for `openssl s_server` depends on the version:
     # * OpenSSL <= 1.0.2a: 512-bit