This change makes TestDialTimeout work on almost all the supported
platforms.

Updates #3016.
Updates #3307.
Updates #3867.
Updates #5380.
Updates #5349.

Change-Id: Iacf0ebea23cdd8f6c0333d70c667a5a5f5eb0ed2
Reviewed-on: https://go-review.googlesource.com/8220Reviewed-by: Ian Lance Taylor <iant@golang.org>