net: close connection in localServer teardown

The transponder sets up a deferred close on accepted connections which
is fine after the client reads all data. However there are no mutexes
nor channels to block the transponder from closing. If the scheduler
runs close before the client read, it will cause an EOF failure.

Fixes #42720

Change-Id: Ic21b476c5efc9265a80a2c6f8484efdb5af66405
Reviewed-on: https://go-review.googlesource.com/c/go/+/273672


Run-TryBot: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Damien Neil <dneil@google.com>