Add tests exercising fundchannel

[dstadulis]

https://github.com/lightninglabs/lightning-terminal/pull/865 corrects the fundchannel functionality to allow a channel to be created of an amount that is less than the cumulative available asset amounts.

MVP TODO

• [ ] test whether multi-input asset funding of a custom channel works

Test-dimension scope to consider:

Positive and negative tests exercising this functionality shall be added asserting channel funding given:

• Single, Multiple asset UTXOs
• A fund_channel size less than, equal, or greater than, the input amounts