This changes rawResponse to install a logger before
Serve()ing and makes the log output available to
tests.

Updates #24831
Updates CL 89275

Change-Id: I0fb636a35b05959ca9978d5d8552f38b7cf8e8b5
Reviewed-on: https://go-review.googlesource.com/106756Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>