net: make two tests not parallel

net: make two tests not parallel

Running

stress -p 1 go test -short std

on a heavily loaded machine causes net timeouts
every 15 or 20 runs.
Making these tests not run in parallel helps.
With this change, I haven’t seen a single failure
in over 100 runs.

Fixes #14986

Change-Id: Ibaa14869ce8d95b00266aee94d62d195927ede68
Reviewed-on: https://go-review.googlesource.com/21905
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>