backend/btc/maketx: don't use btcec in test

[benma]

We want to update the btcd dep, but it heavily refactored the btcec package - it now does not use big.Int anymore and there seems to be no way to get the R and S values as big.Int anymore.

This test is changed to not use the btcec dep at all, circumventing that problem.

I manually converted the DER signature from before to the types.Signature that is needed in the test.