net/http/httptest: redirect example.com requests to server

net/http/httptest: redirect example.com requests to server

The default server cert used by NewServer already includes example.com
in its DNSNames, and by default, the client's RootCA configuration
means it won't trust a response from the real example.com.

Fixes #31054

Change-Id: I0686977e5ffe2c2f22f3fc09a47ee8ecc44765db
Reviewed-on: https://go-review.googlesource.com/c/go/+/666855
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>