net: mark withTCPConnPair as test helper

net: mark withTCPConnPair as test helper

Noticed while analyzing test failures on go.dev/cl/427714 that were
pointing to withTCPConnPair instead of the failing tests.

Change-Id: Ie2366962c2253e0c2a6546d3b7f43a96390bc7bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/428456
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>