Rename a prefixed fullscreen test to full-screen-* for consistency

This way, fullscreen-* can be used for the unprefixed API tests.

BUG=383813

Review URL: https://codereview.chromium.org/440673003

git-svn-id: svn://svn.chromium.org/blink/trunk@179511 bbb929c8-8fbe-4397-9dbb-9b2b20218538