ipv6: skip known-failing tests on OpenBSD 6.8 builders
ipv6: skip known-failing tests on OpenBSD 6.8 builders

These tests are known to be failing on OpenBSD 6.8 and need
investigation. The OpenBSD 6.8 builders are default TryBots
now, so skip them to prevent people working on other packages
from constantly getting a failure.

Issue 42064 remains open to do the work of determining if the
ipv6 package needs to be fixed, or if OpenBSD 6.8 has a problem
and the test can be permanently skipped as unsupported.

For golang/go#11811.
Updates golang/go#42064.
Updates golang/go#35712.

Change-Id: I59883384e4dbce679cb1a4e8e2c3f0fa7983010b
Reviewed-on: https://go-review.googlesource.com/c/net/+/280052
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/net