The SOCK_DGRAM version of the test was introduced for 1.10, and breaks
on older versions.

Fixes golang/go#22899

Change-Id: I97ce6f05a8a885541df823eae996cbf5d87428c2
Reviewed-on: https://go-review.googlesource.com/80275
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>