PP-7211 fix pact state missing parent id

- refund in pact 'state three payments with payout dates exists' is
  missing parent_external_id

src/test/java/uk/gov/pay/ledger/pact/ContractTest.java

@@ -446,6 +446,7 @@ public void createThreePaymentsWithPaidoutDates(Map<String, String> params) {
                 .insert(app.getJdbi());
         aTransactionFixture()
                 .withTransactionType("REFUND")
+                .withParentExternalId(randomAlphanumeric(15))
                 .withGatewayAccountId(gatewayAccountId)
                 .withGatewayPayoutId(gatewayPayoutId4)
                 .insert(app.getJdbi());