ipv4: refactor multicast tests to use subtests
ipv4: refactor multicast tests to use subtests

- Improve error discipline
- Narrow test types from net.UDPAddr to net.IPAddr where possible

Change-Id: I6c403ae69685ac3f35e427f56027fe4a29fde540
Reviewed-on: https://go-review.googlesource.com/c/net/+/260679
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

golang/net