Add tests exercising fundchannel

[dstadulis]

#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

[gijswijs]

#882 fixes this. I'm closing this issue.