[DevTools] Run tests only after all UI bits are initialized.

Tests use WebInspector.panels, which are initialized only in presentUI.

BUG=397648

Review URL: https://codereview.chromium.org/426553003

git-svn-id: svn://svn.chromium.org/blink/trunk@179146 bbb929c8-8fbe-4397-9dbb-9b2b20218538