net/http: handle new HTTP/2 error for 1xx limit exceeded

CL 615295 changed the error message produced by the HTTP/2
implementation when a server sends more 1xx headers than expected.
Update a test that checks for this error.

For #65035

Change-Id: I57e22f6a880412e3a448e58693127540806d5ddb
Reviewed-on: https://go-review.googlesource.com/c/go/+/625195
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/http/transport_test.go

@@ -3278,6 +3278,7 @@ func testTransportLimits1xxResponses(t *testing.T, mode testMode) {
 	for _, want := range []string{
 		"response headers exceeded",
 		"too many 1xx",
+		"header list too large",
 	} {
 		if strings.Contains(err.Error(), want) {
 			return