Fix another case of a parallel test relying on a global variable
(DefaultTransport) implicitly.

Use the private Transport already in scope instead. It's closed at the
end, instead of randomly via another test.

Change-Id: I95e51926177ad19a766cabbb306782ded1bbb59b
Reviewed-on: https://go-review.googlesource.com/32913
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>