go.mod: use github.com/rogpeppe/go-internal/testscript

github.com/rogpeppe/testscript no longer maintain.
We should be use github.com/rogpeppe/go-internal/testscript which still
maintains.
Also, fix filepath.Walk logic on doc/tutorial/basics.TestScript. Might
be related github.com/rogpeppe/go-internal upstream change.

Change-Id: I16ca3f28b9138bf0fbd4cb242ce3f74fc3d66133
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6081
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
diff --git a/internal/cuetxtar/txtar.go b/internal/cuetxtar/txtar.go
index f2b6804..658c818 100644
--- a/internal/cuetxtar/txtar.go
+++ b/internal/cuetxtar/txtar.go
@@ -28,7 +28,7 @@
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/cue/load"
 	"github.com/google/go-cmp/cmp"
-	"github.com/rogpeppe/testscript/txtar"
+	"github.com/rogpeppe/go-internal/txtar"
 )
 
 // A TxTarTest represents a test run that process all CUE tests in the txtar