net/http: fix flaky TestClientRedirect308NoLocation
net/http: fix flaky TestClientRedirect308NoLocation

This was a t.Parallel test but it was using the global DefaultTransport
via the global Get func.

Use a private Transport that won't have its CloseIdleConnections etc
methods called by other tests.

(I hit this flake myself while testing a different change.)

Change-Id: If0665e3e8580ee198f8e5f3079bfaea55f036eca
Reviewed-on: https://go-review.googlesource.com/37624
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>

polyverse-security/go,polyverse-security/go,codestation/go,polyverse-security/go,cloudflare/go,linux-on-ibm-z/go,vsekhar/elastic-go,CAFxX/go,golang/go,akutz/go,CAFxX/go,golang/go,akutz/go,nawawi/go,nawawi/go,nawawi/go,linux-on-ibm-z/go,linux-on-ibm-z/go,ImJasonH/go,polyverse-security/go,josharian/go,CAFxX/go,ImJasonH/go,derekmarcotte/go,CAFxX/go,vsekhar/elastic-go,josharian/go,derekmarcotte/go,Cofyc/go,josharian/go,josharian/go,Cofyc/go,linux-on-ibm-z/go,vsekhar/elastic-go,polyverse-security/go,codestation/go,CAFxX/go,cloudflare/go,ImJasonH/go,codestation/go,derekmarcotte/go,Cofyc/go,josharian/go,Cofyc/go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,codestation/go,vsekhar/elastic-go,akutz/go,akutz/go,golang/go,vsekhar/elastic-go,nawawi/go,nawawi/go,polyverse-security/go,ImJasonH/go,Cofyc/go,nawawi/go,linux-on-ibm-z/go,codestation/go,polyverse-security/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,christopher-henderson/Go,akutz/go,ImJasonH/go,golang/go,cloudflare/go,Cofyc/go,derekmarcotte/go,cloudflare/go,linux-on-ibm-z/go,vsekhar/elastic-go,golang/go,akutz/go,ImJasonH/go,CAFxX/go,derekmarcotte/go,cloudflare/go,derekmarcotte/go,Cofyc/go,ImJasonH/go,josharian/go,CAFxX/go,akutz/go,cloudflare/go,vsekhar/elastic-go,akutz/go,codestation/go,golang/go,vsekhar/elastic-go,cloudflare/go,Cofyc/go,polyverse-security/go,josharian/go,nawawi/go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,ImJasonH/go,golang/go,josharian/go