net/http: avoid leaking the writing goroutine

net/http: avoid leaking the writing goroutine

The test will wait for all goroutines.
A race can occur if the writing goroutine uses the Log after the test exits.

For #58264
For #59883
For #59884

Change-Id: I9b8ec7c9d024ff74b922b69efa438be5a4fa3483
Reviewed-on: https://go-review.googlesource.com/c/go/+/490255
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>