Using variable_get from DrupalWebTestCase
Just faced a similar issue. What I have found out is that in your DrupalWebTestCase child class you still have access to all variables defined in settings.php before calling parent::setUp(). Which means you can do something like this:

class MymoduleWebTestCase extends DrupalWebTestCase {

  public function setUp() {
    $mymodule_test_sftp = variable_get('mymodule_test_sftp');

    parent::setUp();

    variable_set('mymodule_test_sftp', $mymodule_test_sftp);
  }

}