Adapt code to ensure compatibility with latest changes to `Filter.prototype.subscriptions`

Background

As mentioned in trac#6876, the Core team has changed the implementation of Filter.prototype.subscriptions to use Set. That means that we need to change our code accordingly to ensure it remains compatible with the extension.

What to change

• Check and, where necessary, adapt any code that interacts with Filter.prototype.subscriptions.
• Adapt mock layer accordingly.