cmd/cue/cmd: allow "exec" in commands for TestX

Change-Id: I798dee179527fa4fa2c761fbccc0d0b5d903c175
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9343
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>

cmd/cue/cmd/script_test.go

@@ -147,6 +147,9 @@ func TestX(t *testing.T) {
 	for s := bufio.NewScanner(bytes.NewReader(a.Comment)); s.Scan(); {
 		cmd := s.Text()
 		cmd = strings.TrimLeft(cmd, "! ")
+		if strings.HasPrefix(cmd, "exec ") {
+			cmd = cmd[len("exec "):]
+		}
 		if !strings.HasPrefix(cmd, "cue ") {
 			continue
 		}