cmd/dist: ignore stderr when listing packages to test

Currently any warning will make dist fail because the
text will be considered as part of the package list.

Change-Id: I09a14089cd0448c3779e2f767e9356fe3325d8d9
Reviewed-on: https://go-review.googlesource.com/32111
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/dist/test.go

@@ -367,7 +367,7 @@ func (t *tester) registerTests() {
 		if !t.race {
 			cmd.Args = append(cmd.Args, "cmd")
 		}
-		all, err := cmd.CombinedOutput()
+		all, err := cmd.Output()
 		if err != nil {
 			log.Fatalf("Error running go list std cmd: %v, %s", err, all)
 		}