Implement ticket management via GitHub for the unit tests

The WP Unit test framework adds an option to check Trac ticket status for any test and skip the test if the ticket is still open, i.e. unresolved.

This PR builds onto that, by switching out Trac in favour of checking the issue status of a GitHub issue on this repo.

Usage within tests is the same as with the WP Unit test framework - just add a check for knownPluginBug with the issue number at the start of a test:

```php
function someUnitTest() {
	$this->knownPluginBug( $issue_nr );

	// do the actual test
}
```