Rewrite PHPUnit tests to test with constants set/unset

[jazzsequence]

This is a vanity test that doesn't actually test anything that is failing. It was added to test against #30 when the DISABLE_PANTHEON_UPDATE_NOTICES constant is set to true except that at the time the tests are run, it's set to false and the pantheon-updates.php file is loaded. Basically, all the pantheon-updates tests should be inside a separate test group that is only run when the constant is false and separated from other tests when the constant is true. There are likely other constants that perform similar functions and would require similar grouping.