It is very difficult for the fuzzer to create a valid checksum for each
serialized invoice, and we were therefore unable to fuzz deeper than
invoice decoding. We can help the fuzzer generate valid serialized
invoices by calculating and appending the checksum ourselves.
We also switch to using mainnet invoices to make it easier to find valid
invoices for seeding the fuzzer. We prepend the required "lnbc" prefix
ourselves to further help the fuzzer generate valid invoices.
The message signer from invoice_test.go is identical to the one created
in the fuzz test. We're already using the private key from
invoice_test.go, so we may as well use the complete message signer for
simplicity.
The fuzz tests call inv.MinFinalCLTVExpiry() and inv.Expiry() supposedly
to ensure the invoice is well-formed. However, those methods can never
panic or return errors and therefore provide no benefit for this
purpose.
