Use 2048-bit DHE parameters from RFC 3526 instead of 5114 by default

The parameters from RFC 5114 are not considered trustworthy, while those from
RFC 3526 have been generated in a nothing-up-my-sleeve manner.
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index ed695cb..d759752 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -2269,7 +2269,7 @@
                     debug_level=3" \
             0 \
             -c "value of 'DHM: P ' (2048 bits)" \
-            -c "value of 'DHM: G ' (2048 bits)"
+            -c "value of 'DHM: G ' (2 bits)"
 
 run_test    "DHM parameters: other parameters" \
             "$P_SRV dhm_file=data_files/dhparams.pem" \