ipv6: deflake multicast listener tests
ipv6: deflake multicast listener tests

Fixes golang/go#20558.

Change-Id: I8813d1eadd40680bd8619bb9a6539a3c45dcd76e
Reviewed-on: https://go-review.googlesource.com/44770
Run-TryBot: Mikio Hara <e2f4d2eec1d9a1f781345cdd7260d7d52e6468ae@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/net