I have the following test Mnemonic:
stable primary already collect buffalo bike into robust fury cement proof agree neither blouse course
From this I can generate a BIP39 Wallet: https://iancoleman.io/bip39/
From this I can get all the private keys of the derived addresses (if you scroll down to the bottom).
But I'm not sure what format they're in.
The library, bitconjs-lib does the following:
const ONE = Buffer.from('0000000000000000000000000000000000000000000000000000000000000001', 'hex') keyPair = ECPair.fromPrivateKey(ONE)
But the private keys generated by https://iancoleman.io/bip39/ do not match the length required by bitcoinjs-lib