In invalid response tests logger write error messages to stderr and spam
test output.

Since we know response are invalid in these tests we can safely discard
logger output.

Fixes golang/go#22850

Change-Id: Id8c97be910f0cf7dbe2380ba632960364bc8478b
Reviewed-on: https://go-review.googlesource.com/80235Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>