ipv6: re-enable tests on Dragonfly
ipv6: re-enable tests on Dragonfly

Now that golang.org/x/net was re-vendored into std, these tests should
pass again.

Updates golang/go#34368

Change-Id: I6f253896836fb18d46875a9420de58ca05a49646
Reviewed-on: https://go-review.googlesource.com/c/net/+/202457
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net