net/http: deflake TestClientRedirect308NoGetBody

net/http: deflake TestClientRedirect308NoGetBody

In an unrelated CL I found a way to increase the likelihood of latent
flaky tests and found this one.

This is just like yesterday's https://golang.org/cl/37624 and dozens
before it (all remnants from the great net/http test parallelization
of Nov 2016 in https://golang.org/cl/32684).

Change-Id: I3fe61d1645062e5109206ff27d74f573ef6ebb2e
Reviewed-on: https://go-review.googlesource.com/37627
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>