net/http: deflake TestClientRedirectTypes and maybe some similar ones

net/http: deflake TestClientRedirectTypes and maybe some similar ones

A few tests were using the global DefaultTransport implicitly.
Be explicit instead. And then make some parallel while I'm there.

Change-Id: I3c617e75429ecc8f6d23567d1470f5e5d0cb7cfd
Reviewed-on: https://go-review.googlesource.com/32758
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>