http2: don't log expected errors from writing invalid trailers

Change-Id: I1c8af5a1f7539a25d5602a7bc8e15756d3cafa56
Reviewed-on: https://go-review.googlesource.com/c/net/+/653695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

http2/server_test.go

@@ -2814,6 +2814,8 @@ func testServerWritesTrailers(t *testing.T, withFlush bool) {
 		w.Header().Set("Trailer", "should not be included; Forbidden by RFC 7230 4.1.2")
 		return nil
 	}, func(st *serverTester) {
+		// Ignore errors from writing invalid trailers.
+		st.h1server.ErrorLog = log.New(io.Discard, "", 0)
 		getSlash(st)
 		st.wantHeaders(wantHeader{
 			streamID:  1,