Test the failure cases, not just the happy one

# Sandbox keys are prefixed sk_test and never move real money.
export SYNTRA_KEY=sk_test_4f2a9c...

curl https://sandbox.syntrapayments.com/v1/payouts \
  -H "Authorization: Bearer $SYNTRA_KEY" \
  -H "Idempotency-Key: first-test-payout" \
  -d source_currency=NGN \
  -d destination_currency=USD \
  -d amount=1000.01 \
  -d beneficiary=bnf_test_001

{
  "id": "po_test_9Xa",
  "status": "failed",
  "failure_reason": "invalid_account"
}