This changes the default behavior to always check wordpress.org for the latest information when doing wp plugin|theme list unless the existing --skip-update-check flag is passed.

This came out of the discussion in #426 and replaces that PR. The main idea is that it doesn't make any sense to have both --force-check and --skip-update-check flags while also having the default behavior be unpredictable depending on the state of the transient.

I created a new PR since pretty much all of the code is test code and that is different now that we are changing the default behavior instead of adding a flag.