loopback: enable logger for tests

Enabling the logger shows this helpful warning

  warning: Inode.Path: n3 is orphaned, replacing segment with ".go-fuse.5577006791947779410/deleted"

for the problem reported at https://github.com/hanwen/go-fuse/issues/398 ,
and it generally makes sense to have complete logs for tests.

Enable the logger.

Change-Id: I4602f100a31c6e50bc48251a877b97ce3013ff24

fs/simple_test.go

@@ -104,6 +104,7 @@ func newTestCase(t *testing.T, opts *testOptions) *testCase {
 	tc.rawFS = NewNodeFS(tc.loopback, &Options{
 		EntryTimeout: entryDT,
 		AttrTimeout:  attrDT,
+		Logger:       log.New(os.Stderr, "", 0),
 	})
 
 	mOpts := &fuse.MountOptions{}