net/http: test intended behavior in TestClientInsecureTransport

This test wasn't testing the HTTP/2 case, because it didn't
set NextProtos in the tls.Config.

Set "Connection: close" on requests to make sure each request
gets a new connection.

Change-Id: I1ef470e7433a602ce88da7bd7eeec502687ea857
Reviewed-on: https://go-review.googlesource.com/c/go/+/655676
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

Author: neild

src/net/http/client_test.go

@@ -834,8 +834,11 @@ func testClientInsecureTransport(t *testing.T, mode testMode) {
 	for _, insecure := range []bool{true, false} {
 		c.Transport.(*Transport).TLSClientConfig = &tls.Config{
 			InsecureSkipVerify: insecure,
+			NextProtos:         cst.tr.TLSClientConfig.NextProtos,
 		}
-		res, err := c.Get(ts.URL)
+		req, _ := NewRequest("GET", ts.URL, nil)
+		req.Header.Set("Connection", "close") // don't reuse this connection
+		res, err := c.Do(req)
 		if (err == nil) != insecure {
 			t.Errorf("insecure=%v: got unexpected err=%v", insecure, err)
 		}