git-codereview: specify a dummy cookie file
git-codereview: specify a dummy cookie file

This test was incorrectly passing when it should be dying, because
I have a global http.cookiefile defined that this test falls back to
using if a local http.cookiefile isn't configured.

Fix this by defining a local, invalid cookiefile, which causes
git/git-codereview to error out.

Change-Id: Ia538d3e8af53b849d672419582c21ba8a6fdbbdd
Reviewed-on: https://go-review.googlesource.com/49293
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/review