cmd/cue/cmd: auto-detect test mode

Closes #149
https://github.com/cuelang/cue/pull/149

GitOrigin-RevId: 747327ff8b475c015179f5dd52f762e62d5e9a7b
Change-Id: I0a1e363c7058d29494a5c56eae7f275644650382
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3546
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cmd/cue/cmd/common.go b/cmd/cue/cmd/common.go
index 2cec1e8..a2b9c7b 100644
--- a/cmd/cue/cmd/common.go
+++ b/cmd/cue/cmd/common.go
@@ -16,6 +16,7 @@
 
 import (
 	"bytes"
+	"flag"
 	"io"
 	"os"
 	"strings"
@@ -34,6 +35,12 @@
 
 var inTest = false
 
+func init() {
+	if flag.Lookup("test.v") != nil || strings.Contains(os.Args[0], "/_test/") {
+		inTest = true
+	}
+}
+
 func mustParseFlags(t *testing.T, cmd *cobra.Command, flags ...string) {
 	if err := cmd.ParseFlags(flags); err != nil {
 		t.Fatal(err)