cmd/bundle: expand test coverage to all modes

As of CL 189818, bundle has been updated to use the go/packages API to
load packages. That API supports module mode and legacy GOPATH mode.
Update the test to provide coverage for all modes.

Updates golang/go#24661.

Change-Id: Ied6196f7317b9b0289faf80dbfe9815e9b98b55d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/255053
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

cmd/bundle/main_test.go

@@ -15,7 +15,8 @@ import (
 	"golang.org/x/tools/go/packages/packagestest"
 )
 
-func TestBundle(t *testing.T) {
+func TestBundle(t *testing.T) { packagestest.TestAll(t, testBundle) }
+func testBundle(t *testing.T, x packagestest.Exporter) {
 	load := func(name string) string {
 		data, err := ioutil.ReadFile(name)
 		if err != nil {
@@ -24,7 +25,7 @@ func TestBundle(t *testing.T) {
 		return string(data)
 	}
 
-	e := packagestest.Export(t, packagestest.Modules, []packagestest.Module{
+	e := packagestest.Export(t, x, []packagestest.Module{
 		{
 			Name: "initial",
 			Files: map[string]interface{}{