Using Iperf version 2.0.9 on Debian (Stretch) it’s possible to test networks using a multicast connection.

On the receiving end execute the command:
iperf -s -u -B 239.1.1.10 -i 1

On the sending end execute the command:
iperf -c 239.1.1.10 -u -T 3 -t 10 -i 1 -b 100M

This will run a bandwidth test using UDP traffic at 100Mbps.

Note, if your system is multi-homed you must make sure your multicast traffic is routed out of the correct interface. For example:
ip route add 224.0.0.0/4 dev eth0