[release-branch.go1.13] net: add more timing slop for TestDialParallel on Windows

For #35616.
Fixes #39538.
For #29252.

Change-Id: I51b2490100cfe0e902da09eee8d027e0ec86ed53
Reviewed-on: https://go-review.googlesource.com/c/go/+/207466


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit c20b71eb)
Reviewed-on: https://go-review.googlesource.com/c/go/+/237602


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>