fees: add tests to validate that non-zero fees are working

[TalDerei]

Create unit tests to verify correctness of non-zero fees in the context of properly balancing transactions. There are a few cases to consider:

• single large note > amount + cumulative fees --> success
• single small note < amount + cumulative fees --> failure
• multiple smaller notes > amount + cumulative fees --> success
• multiple smaller notes < amount + cumulative fees --> failure
• exact note for amount + another large note > amount + cumulative fees --> success
• exact note for amount + another small note < amount + cumulative fees --> failure

[TalDerei]

tagging penumbra-zone/web#690 for visibility.