Implement cancelInvoice integration tests

[gabrielstoica]

There's a current blocker in integrating the integration tests for the cancelInvoice method as any stream created through the StreamManager (part of the InvoiceModule) contract will be owned by the contract itself and not by the payer.

This can cause front-end issues because we'll not be able to display streams by payers but rather by InvoiceModule.

An ongoing discussion with the Sablier team has been opened here.

EDIT: After discussing with the Sablier team, the only workaround is to add a mapping in the StreamManager so we can map the initial stream sender with the stream ID. Therefore, each call to either cancelLinearStream or cancelTranchedStream is guarded by an OnlyInitialStreamSender checker.

On the front-end side, the InvoiceModule is going to emit an InvoicePaid event each time a new stream is created which gives us the ability to map the initial stream sender with the stream ID off-chain through the MetaDock subgraph.