net/http: deflake TestClientRedirects

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>

src/net/http/client_test.go

@@ -251,11 +251,14 @@ func TestClientRedirects(t *testing.T) {
 	var checkErr error
 	var lastVia []*Request
 	var lastReq *Request
-	c = &Client{CheckRedirect: func(req *Request, via []*Request) error {
+	c = &Client{
+		Transport: tr,
+		CheckRedirect: func(req *Request, via []*Request) error {
 			lastReq = req
 			lastVia = via
 			return checkErr
-	}}
+		},
+	}
 	res, err := c.Get(ts.URL)
 	if err != nil {
 		t.Fatalf("Get error: %v", err)