Improve the tests, move from hourly to monthly intervals

Our tests currently use hourly intervals. #3038991: Prorating logic is not correct for prepaid subscriptions has shown that hourly intervals can hide bugs, since they produce billing periods that are always equal, unlike monthly ones.

To prepare for fixing that issue, let's convert our tests to use monthly intervals.

Let's also move SubscriptionActivateTest to Plugin\AdvancedQueue\JobType\SubscriptionActivateTest, since the test class should match the namespace of the class it is testing.