encoding/openapi: fix updating golden files in tests

When `CUE_UPDATE` is true, the test should not
fail when the test output differs, but it does currently.
This fixes that.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I8b304c17a9252f0e983aa623aeea3a1aca9d346d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1205770
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

encoding/openapi/decode_test.go

@@ -116,7 +116,6 @@ func TestDecode(t *testing.T) {
 				out = bytes.TrimSpace(out)
 
 				if diff := cmp.Diff(b, out); diff != "" {
-					t.Error(diff)
 					if cuetest.UpdateGoldenFiles {
 						a.Files[outIndex].Data = b
 						b = txtar.Format(a)