tests: Include stderr in TestGopherJSCanBeVendored output.

This way, we get more information when/if TestGopherJSCanBeVendored
fails.

Before:

	$ go test -v -run=TestGopherJSCanBeVendored
	=== RUN   TestGopherJSCanBeVendored
	--- FAIL: TestGopherJSCanBeVendored (5.20s)
		gorepo_test.go:38: exit status 1
	FAIL
	exit status 1
	FAIL	github.com/gopherjs/gopherjs/tests	5.203s

After:

	$ go test -v -run=TestGopherJSCanBeVendored
	=== RUN   TestGopherJSCanBeVendored
	vendoring github.com/gopherjs/gopherjs/js package is not supported, see #415
	--- FAIL: TestGopherJSCanBeVendored (5.19s)
		gorepo_test.go:40: exit status 1
	FAIL
	exit status 1
	FAIL	github.com/gopherjs/gopherjs/tests	5.190s

tests/gorepo_test.go

@@ -33,7 +33,9 @@ func TestGopherJSCanBeVendored(t *testing.T) {
 		t.Skip("test meant to be run using normal Go compiler (needs os/exec)")
 	}
 
-	got, err := exec.Command("bash", "gopherjsvendored_test.sh").Output()
+	cmd := exec.Command("bash", "gopherjsvendored_test.sh")
+	cmd.Stderr = os.Stdout
+	got, err := cmd.Output()
 	if err != nil {
 		t.Fatal(err)
 	}