net: enable multicast listener tests on solaris/illumos

net: enable multicast listener tests on solaris/illumos

It seems everything is in place for these tests to pass on solaris and
illumos, so enable them.

Fixes #7399

Change-Id: If6defb651ef9c5059c1aeccbc4fc13a12a86b682
Reviewed-on: https://go-review.googlesource.com/c/go/+/346149
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>