net/http: convert TestSetsRemoteAddr to use clientServerTest

net/http: convert TestSetsRemoteAddr to use clientServerTest

This is an example of converting an old HTTP/1-only test to test
against both HTTP/1 and HTTP/2.

Please send more of these!

Also, for comparing the http.Transport's responses between HTTP/1 and
HTTP/2, see clientserver_test.go's h12Compare type and tests using
h12Compare. Sometimes that's the more appropriate option.

Change-Id: Iea24d844481efd5849173b60e15dcc561a32b88f
Reviewed-on: https://go-review.googlesource.com/17409
Reviewed-by: Burcu Dogan <jbd@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>