Test that cancel returns the refunded amount

[Glittersup]

Summary

cancel returns the amount refunded to the sender. The tests verify the sender's balance afterwards but not the returned value, which is what a caller integrating programmatically would use.

Acceptance criteria

• A test asserts the value returned by cancel equals the refund to the sender.
• The assertion holds for a stream cancelled part way through.
• The suite still passes.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/tricklepay-contracts.git
cd tricklepay-contracts
git remote add upstream https://github.com/TricklePay/tricklepay-contracts.git

Create a branch for this issue:

git checkout -b test/issue-183

Suggested commit message:

test: assert the value returned by cancel

Run cargo fmt --check, cargo clippy --all-targets -- -D warnings, and cargo test before opening a pull request and linking this issue.