net/http/httputil: fix deadlock in DumpRequestOut

Fix a deadlock in DumpRequestOut which can occur if the request is
cancelled between response being sent and it being processed.

Also:
* Ensure we don't get a reader leak when an error is reported by the
  transport before the body is consumed.
* Add leaked goroutine retries to avoid false test failures.

Fixes #38352

Change-Id: I83710791b2985b997f61fe5b49eadee0bb51bdee
Reviewed-on: https://go-review.googlesource.com/c/go/+/232798


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Damien Neil <dneil@google.com>