title: Flutter Widget Tests with NetworkImage
body: <p>I have a <code>Widget</code> with <code>NetworkImage</code> (so far with hard-coded url).<br>
  I would like to widget test this Widget, but I got 404 when I run widget test (url is 100% valid).<br>
  How can I make <code>NetworkImages</code> load themselves or (which would be better) <strong>ignore them</strong> so that my tests won't fail because of 404? </p>