Add BallotTest class, which should extend MediaWikiUnitTestCase.

The source file should be located in SecurePoll/tests/phpunit/unit and namespaced to MediaWiki\Extensions\SecurePoll\Test\Unit.

This will involve some judgement as to what is worth testing, and what can be tested in a unit test.

We should consider whether it would be helpful to make this test extensible, as a base class for tests for the Ballot subclasses. If extra work is required to do this, we can file it as a follow-up.