I thought that we avoided creating on-disk Unix sockets,
but I was mistaken. Use one to test CL 17458.

Fixes #11826.

Change-Id: Iaa1fb007b95fa6be48200586522a6d4789ecd346
Reviewed-on: https://go-review.googlesource.com/17725
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>