net/http: ignore ECONNRESET errors in TestTransportConcurrency on netbsd

net/http: ignore ECONNRESET errors in TestTransportConcurrency on netbsd

The source of these errors is undiagnosed, but they have only been
observed on netbsd builders (on a variety of architectures).

Tested manually by injecting this code into the test's handler:

		if mrand.Intn(4) == 0 {
			if conn, _, err := w.(Hijacker).Hijack(); err == nil {
				conn.(*net.TCPConn).SetLinger(0)
				conn.Close()
				return
			}
		}

and temporarily disabling the 'runtime.GOOS' part of the condition.

For #52168.

Change-Id: I10965803e5a0d493ac4a000575de8b5f0266989c
Reviewed-on: https://go-review.googlesource.com/c/go/+/398635
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>