ve.isMixedIn: Fix bugs and add unit tests

* It returned undefined instead of false if the subject
  didn't have a 'mixins' property. This is because '&&'
  is a DEFAULT operator, not an AND operator, it returns the
  value, not a boolean per se.
* The logic of traversing to the constructor property was
  broken since all objects in javascript have a constructor
  property, and functions/constructors are also objects.

Follows-up Ic3e4472b9e694.

Change-Id: I462e7ce270c8cfc7e1970e359894ee4b7d90b881