Add extra, negative case to unit tests

[dstadulis]

Additional edge cases need coverage, for which a new PR will be added imminently
Potential solution:

Add extra, negative case to unit tests (https://github.com/lightninglabs/lightning-terminal/pull/842/files)

Details:

• If node attempts a send of an amount that maps to more than the total payment bandwidth, the payment should fail
• Explore more boundary edge cases by increasing property-based tests [EDIT: Added in this commit]