message: snapshotter/test: no need to validate "cache" subdir is created
  The cache store directory is specified in nydus config.json already.
  Nydus is going to create the work_dir recursively. So snapshotter
  does not have to create it and validate it.
diff: @@ -109,6 +109,5 @@ func Test_filesystem_generateDaemonConfig(t *testing.T) {
  })
  require.Nil(t, err)
  assert.Nil(t, ensureExists(filepath.Join(snapshotRoot, "config", d.ID, "config.json")))
  - assert.Nil(t, ensureExists(filepath.Join(snapshotRoot, "cache")))
  assert.Nil(t, ensureExists(filepath.Join(snapshotRoot, "socket", d.ID)))
  }
  