Store the last package in Preferences.sublime-settings.

Hello! I made a small change so that the name of the last package, which was used for unit testing, is saved in the user's preferences. This avoids the usage of a global variable and is persistent so that the input box can be prefilled even if Sublime Text was restarted.

It would be even better, if one could save the settings in a session-based storage but unfortunately this is not possible without further ado (see this [link](http://sublimetext.userecho.com/topic/101207-api-to-access-a-projects-sublime-project-and-sublime-workspace-files/)).