DevTools: make tests preload panels based on their folder instead of fetching them lazily.

R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/619653002

git-svn-id: svn://svn.chromium.org/blink/trunk@183031 bbb929c8-8fbe-4397-9dbb-9b2b20218538