http2: add missing test update from CL 200317

I forgot to run the trybots on my CL 200317 and also forgot to include
the test update. Whoops.

Change-Id: I5453f6a7314daa49429b6a3ee49a1b5710c05803
Reviewed-on: https://go-review.googlesource.com/c/net/+/206217
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

http2/server_test.go

@@ -3269,7 +3269,7 @@ func TestConfigureServer(t *testing.T) {
 			tlsConfig: &tls.Config{
 				CipherSuites: []uint16{tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384},
 			},
-			wantErr: "is missing an HTTP/2-required AES_128_GCM_SHA256 cipher.",
+			wantErr: "is missing an HTTP/2-required",
 		},
 		{
 			name: "required after bad",