net/http: add tests for Server.ReadTimeout and server.WriteTimeout

net/http: add tests for Server.ReadTimeout and server.WriteTimeout

We don't seem to have tests verifying that handler reads from the
request body or writes to the response body time out properly.
Add some.

For #49837
For #56478

Change-Id: I0828edd6c86b071073fd1b22ccbb24f86114ab94
Reviewed-on: https://go-review.googlesource.com/c/go/+/446255
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>