CB-7459 Allow automatic tests to be run for specific plugin(s)

Implementation for https://issues.apache.org/jira/browse/CB-7459

See comments in commits for more details.
One concern here: if any other code adjusts the "enabled" state of the cdvtests.tests modules, but forgets to update the localStorage entry, then that value will get overwritten here.

Perhaps now that we are adding localStorage backing, we should add methods to cdvtests module for `isEnabled(key)` and `setEnabled(key, bool)`?