Add tests to cover BlogPost::add_default_author config

[michalkleiner]

#669 added a new config to control whether the current member is added as the default author for a newly created blog post.

Likely needs to live within/extend https://github.com/silverstripe/silverstripe-blog/blob/3/tests/php/BlogPostTest.php.

PRs

• #671

[emteknetnz]

PR #671