MDL-58462 mod_data: Generate users with different names

MDL-58462 mod_data: Generate users with different names

* Give test users different first and last names.
* Declare class member variables instead of dynamically assigning them.
* Rename member variable $data to $database since it conflicts with the
  private member variable PHPUnit_Framework_TestCase::$data.
* Replace setExpectedException usage with expectException().