Ensure that CustomMessageHandler impl is tested

[luckysori]

With #128 we introduced changes to the messages sent between the two parties involved in creating LN-DLC channels (probably for vanilla DLC channels too?). The tests in ln_dlc_channel_execution_tests.rs passed, but using the library revealed that we had not updated the CustomMessageHandler implementation to ensure that LDK would be able to forward the new custom message to us. For this reason we had to open #131.

Originally posted by @Tibo-lg in #131 (review)