net/http: fix parallel tests using global DefaultTransport

net/http: fix parallel tests using global DefaultTransport

When I added t.Parallel to some tests earlier, I overlooked some using
the global "Get" func, which uses DefaultTransport.

The DefaultTransport can have its CloseIdleConnections called by other
parallel tests. Use a private Transport instead.

Fixes #18006

Change-Id: Ia4faca5bac235cfa95dcf2703c25f3627112a5e9
Reviewed-on: https://go-review.googlesource.com/33432
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>