internal/cuetxtar: allow CUE_UPDATE env variable.

Not all package implement the update flag. Using an
env var allows selecting update when using
go test ./...

Change-Id: Ic1eaae1dc5c36b7f3c9cf8692cdc3f68b5d1d970
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6581
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

internal/cuetxtar/txtar.go

@@ -31,6 +31,8 @@ import (
 	"github.com/rogpeppe/go-internal/txtar"
 )
 
+var envUpdate = os.Getenv("CUE_UPDATE")
+
 // A TxTarTest represents a test run that process all CUE tests in the txtar
 // format rooted in a given directory.
 type TxTarTest struct {
@@ -245,7 +247,7 @@ func (x *TxTarTest) Run(t *testing.T, f func(tc *Test)) {
 				return
 			}
 
-			if !x.Update {
+			if !x.Update && envUpdate == "" {
 				t.Fatal(cmp.Diff(string(gold.Data), string(result)))
 			}