Comment tests using story as node type + another small fix

The comment tests were still using a 'story' node type which doesn't ship with drupal Core anymore. Fixed in the patch.

The comment tests currently has 18 fails. This is because of this part:

    $anonymous_comment3 = $this->postComment($this->node, $this->randomName(), $this->randomName(), TRUE, TRUE);
    $this->assertText(t('E-mail field is required.'), t('E-mail required.')); // Name should have 'Anonymous' for value by default.
    $this->assertFalse($this->commentExists($anonymous_comment3), t('Anonymous comment with contact info (required) not found.'));

    // Post comment with contact info (required).
    $anonymous_comment3 = $this->postComment($this->node, $this->randomName(), $this->randomName(), FALSE, array('mail' => 'tester@simpletest.org'));
    $this->assertTrue($this->commentExists($anonymous_comment3), t('Anonymous comment with contact info (required) found.'));

The first postComment threats the comment with preview = TRUE and the second one used preview = FALSE. But the setting for preview doesn't get flipped in the middle. It is fixed in this patch by using TRUE for the second time too.