Fix a test.
Fix a test.

Calling Bytes() on a big.Int strips any leading padding zeros.  This
change fixes the test to always pad the byte slice for a private key
to a length of 32.

btcsuite/btcwallet,synrg-labs/btcwallet,Stash-Crypto/btcwallet,synrg-labs/btcwallet,jrick/dcrwallet,jrick/btcwallet,Roasbeef/btcwallet,Roasbeef/btcwallet,jrick/btcwallet,decred/dcrwallet,jrick/dcrwallet,Stash-Crypto/btcwallet,decred/dcrwallet,btcsuite/btcwallet