Add tests for JSON-RPC filter and subscription middleware

[Gudahtt]

We should have tests for the subscription and filter middleware which were moved into the network client as part of #16693

This is important for validating the behavior of the network client before we migrate it to the mobile network controller.

[Gudahtt]

Putting this issue aside for now, as I discovered that it depends upon MetaMask/core#1047

[Gudahtt]

I decided that this isn't blocked after all, as I can update the network client tests to bring in the filter and subscribe middleware to the client under test ahead of that refactor

[kenhkan]

Mark and I met up and we decided that this is not necessary for the multichain workstream 1 delivery and the SL team will decide on what to do with this ticket at a later date.