book-and-seller-replace-items test should assert that capacity changes as expected after a replacement

[lukehesluke]

If a booked opportunity is replaced for another one, then the capacity of the old one should go back up and the capacity of the new one should go down by one

Notes to implementer

Look at packages/openactive-integration-tests/test/features/cancellation/seller-requested-replacement/implemented/book-and-seller-replace-items-test.js

You can see that there's some commented out code that starts to implement this capacity assertion, but it's commented out. I don't know why it's commented out. Some options:

• It doesn't work because there's an issue with OpenActive.Server.NET's BookingSystem.AspNetCore. In which case, fix that
• It was commented out because the logic is tricky and it was incomplete. In which case, fix this