Improve TxSummaryCollector

[ImplOfAnImpl]

• If an asset is only present in total_outputs, the summary collector will currently not fail with TxFeeUnderflow, but it should.
  Addressed by Fixing issues #23.
• TxSummaryCollector's tests can be improved.

See the corresponding TODOs in crates/app-core/src/handlers/sign_tx/summary_collector.rs.