net/http: fix flaky TestDisableKeepAliveUpgrade

This test hijacks a connection. It was reading from the net.Conn
returned by Hijack, not the bufio.ReadWriter, causing flaky failures
when a read-ahead byte was held in the read buffer.

Fixes #43073.

Change-Id: Ic3e7f704fba9635fd851cb3c0c0c74e312b75f6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/285596


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Anmol Sethi <nhooyr@gmail.com>